The Alexandria deputy sheriff who left a woman locked in a tiny courthouse cell last weekend resigned yesterday.

Sheriff James H. Dunning said the deputy, Vernell Bolton, forget she had locked Cassandra Jones in the 8-by-11-foot room on April 18.

Jones, who was supposed to serve only 3 1/2 hours in jail, was found sitting on the cell's wooden bench during a routine check the following Monday. She had passed the weekend without food, clean water or a bed to sleep in.

"Deputy Bolton was the subject of a disciplinary investigation and she chose to resign," said Dunning. Bolton was unavailable for comment.

Jones, a 25-year-old Prince George's County resident who pleaded guilty to a misdemeanor in Alexandria General District Court, has said she is considering legal action against the sheriff's office. Because he is an elected officer of the state, Dunning is insured through a state liability fund.

Dunning said it is routine practice for each prisoner to be assigned to a deputy who is responsible for checking the person in and out of the holding cell.

Dunning, who was elected last year after promising to improve management and security at the city jail, said that he was still unsure why the cell was not checked each night by a duty officer, and that the investigation into the incident was continuing.

The Jones incident was the second major security mixup for the sheriff's office in the past month. On April 3, Tyrone McIlwaine, wanted in Maryland on a fugitive warrant, escaped from the Alexandria Courthouse while wearing leg irons. McIlwaine had escaped earlier in the same week from a cell at D.C. Superior Court.