The predaceous mosquito Toxorhynchites rutilus septentrionalis (Dyar and Knab) was colonized by induced copulation and reared under controlled laboratory conditions to obtain data on each stage of the life cycle. Eggs hatched 3.0 days after oviposition and the following mean values were obtained for larvae which emerged as females when reared at 27 C: 1st instar 1.6 days, 2nd instar 2.0, 3rd instar 3.2, 4th instar 6.6 pupa 5.4. Total development from oviposition to adult emergence required 21.8 days (20.6-22.6) for females; males completed their development approximately 1 day sooner. When fed with 1st and 3rd instar prey, female Tx. r. septentrionalis devoured 237.8 (222-245) Ae. aegypti larvae during their development. The males consumed fewer prey but killed a greater number without consuming them. Killing behavior was first evident in male larvae 4 days before pupation and increased daily until more than 50% of the available prey were destroyed on the day prior to pupation. Female larvae exhibited some killing behavior but did not hunt and kill prey as actively as the males. The biological advantage of sex-related compulsive killing is discussed.