The morphology of mitotic chromosomes from brain tissues of fourth-instar larvae has been studied in 12 species of mosquitoes. These included an unidentified species of Corethra, Anopheles quadrimaculatus Say, Wyeomyia smithii Coquillett, Culex pipiens pipiens Linn., C. restuans Theobald, C. territans Walker, Aedes togoi Theobald, A. vexans Meigen, A. albopictus Skuse, A. aegypti Linn., A. atropalpus Coquillett and A. stimulans Walker. The diploid chromosome number of all species examined except Corethra (2n=8) is six. Based on their length and the position of the centromere, individual chromosomes have been numbered from I to III (I to IV in Corethra). Chromosome I is the smallest chromosome in the complement and is dimorphic in Corethra sp. and Anopheles quadrimaculatus. With the possible exception of Culex pipiens pipiens and one of the members of this pair in the males of Corethra and Anopheles, chromosome I is metacentric. The next larger chromosome has been designated as chromosome II. It is submetacentric in Aedes aegypti, A. atropalpus and A. stimulans. In all other species it is metacentric. Chromosome III which is the largest of the three chromosomes is submetacentric in A. stimulans and metacentric in others. The size of chromosome I is very variable in different species and so is the ratio of chromosome I to chromosomes II and III. This ratio is lower in Culex than in Aedes. The early prophase chromosomes of Wyeomyia smithii are usually kinky and coiled. Towards the end of its shorter arm, chromosome II in Aedes atropalpus shows an achromatic gap in one or both of its liomologues. Each resting nucleus in this species shows two conspicuous Feulgen-positive particles at opposite poles. One particle was seen in each resting nucleus of Anopheles quadrimaculatus and Corethra sp. Rarely polyploid cells were observed in the brain tissues of Wyeomyia smithii and Culex pipiens pipiens.