A co-tenant who wants to leave in the middle of a tenancy is legally responsible, if she is a month-to-month tenant, for giving the landlord proper written notice and paying rent through the end of the notice period. If there's a lease, the tenant should either get permission from the landlord to leave early or, if this is impossible, find a new tenant who is acceptable to the landlord. If a co-tenant simply leaves, the fallout can be serious.

The unauthorized departure of a co-tenant gives the landlord the option of evicting the rest of you, even if you are able to pay the full rent. The landlord has this option because breaking the lease or rental agreement by even one tenant is a violation of a key lease term (the length of stay), for which all tenants are liable.

In practice, however, your landlord will probably let you stay if it will keep a steady stream of rent money coming in and keep the place occupied by stable, non-destructive tenants. So if you pay the rent after a co-tenant has broken the lease and left, the landlord will probably not evict you and other tenants unless:

• you are a troublesome tenant, and this is a golden opportunity to be rid of you, or

• your income doesn't appear sufficient to cover the rent in the future. In this case, if you can assure the landlord that you can promptly bring in a good, law- and lease-abiding new co-tenant, you might be able to salvage your tenancy. In the meantime, you may need to ask permission to pay the rent late or in installments. Or, ask the landlord to use the departed tenant's share of the security deposit to help pay the rent until you find an acceptable replacement.

If a co-tenant takes off and leaves you facing the entire rent, you may be tempted to simply move in another roommate, bypassing the landlord's application process. Don't! Your lease or rental agreement probably prohibits unauthorized sublets. If it does, bringing in a new tenant -- even a great one -- without your landlord's okay violates your agreement and gives your landlord a watertight reason to evict you. Instead, keep your relationship on an honest footing and get your landlord's approval for a replacement tenant.

Remaining roommates need to cover their legal flanks with respect to the departed tenant as well as the landlord. If your housemate has left during the middle of a lease or without proper notice in a month-to-month tenancy, leaving you responsible for all the rent, your personal relations will be rocky at best. Probably the last thing you want is to have your errant roommate reappear expecting to move back in.

To avoid such surprises, try to get your former roommate to sign an agreement, making it clear that the departing tenant:

• Will pay a stated amount of rent and utilities. If you rent under a written rental agreement, this will normally be rent and utilities for 30 days from the date the departing tenant gave written notice (or left without notice) unless a new roommate comes in earlier and covers these costs. If you rent under a lease, the amount owed will depend on when a new co-tenant, acceptable to the landlord, is ready to take over. If, despite your best efforts, you cannot find an acceptable replacement, the departing tenant will be liable for the rent for the balance of the lease.

• Will pay for any damage she caused to the rental unit.

• Will pay for rent and damage no later than a stated date.

• Has moved out for good and gives up any claim to be a tenant.

But what if you and the departing roommate can't work things out, and the departed co-tenant shows no signs of paying? If your roommate is long gone or out-of-state, you may want to grit your teeth, pay his share and forget it, since trying to find him, sue him and then collect the judgment is likely to be more trouble than it's worth.

On the other hand, if your ex-roommate is still in town and has a source of income, consider taking the time to sue him in small claims court for unpaid rent, damage to the rental unit, unpaid utilities and your costs to find a replacement co-tenant, such as advertising. Then, if your ex-roommate still doesn't pay up, you can collect what you won in court from his bank account or wages.

If your co-tenant skips out, leaving you in the lurch, you may decide that it's not worth the hassle of trying to stay and rustle up another roommate.

To ease your departure and forestall the landlord from keeping your security deposit to make up for unpaid rent, or listing you as a deadbeat at the credit bureau, follow these steps:

• If you are a month-to-month tenant, give the required amount of written notice (usually 30 days) immediately. Don't wait until you can't pay the next month's rent and receive a termination notice.

• If you have a lease, let the landlord know in writing that you plan to move because you cannot afford the rent without your co-tenant. Before you move, be extra accommodating when it comes to showing the unit to prospective renters. Facilitating a quick re-rental is not just a courtesy to your landlord, but to your advantage as well, since the sooner a new tenant takes over, the sooner your liability for the balance of the rent due under the lease ends. In addition, do your best to find an acceptable replacement tenant yourself.