As in previous limited entomological studies carried out toward the end of the 1969 yellow fever epidemic on the Jos Plateau, Nigeria, the vector mosquito was not identified, additional observations were made in April, June, August-September and October 1970. In the three study areas selected, each the scene of yellow fever cases in 1969, special attention was given to the larval habitats and adult biting cycles of five species of Aedes (Stegomyia). Holes in the broken or cut stems of Euphorbia kamerunica, a plant commonly used as hedge in rural settlements, proved to be well utilised by larvae of Ae. aegypti (L.), Ae. africanus (Theo.) and Ae. luteocephalus (Newst.), the latter species being the most abundant. Collections of adults on human bait, mainly from 12.00 to 21.00 h, showed that species of this subgenus comprised between 70 and 89% of all mosquitoes captured in June-October. Ae. luteocephalus always predominated, followed by Ae. aegypti, then Ae. vittatus (Big.), Ae. africanus and Ae. simpsoni (Theo.). Ae. aegypti was found not to be a domestic, household-inhabiting species in the study areas. The indications are that Ae. simpsoni is not strongly anthropophilic on the plateau. Two arboviruses, UgMP 359 and a member of the Bunyamwera group, were isolated, from Ae. (Aedimorphus) dentatus (Theo.) and Anopheles funestus Giles, respec tively; no isolations were made from 2033 adults of Stegomyia.