This paper presents the results of obsrvation on the circadian biting cycle of mosquitoes, at Granjas Calábria, Jacarepaguá, in the coastal lowland of Rio de Janeiro, Brazil, where we carried out weekly outdoor captures on human bait, from 8 to 10 a.m. and from 1 to 3 and 6 to 8 p.m., from August 1981 to July 1982, as well as three nyctemeral captures also on human bait. Most of the local species were more frequent at sunset and at night. Li. durhami, Ph. davisi, Wy. leucostigma and Wyeomyia (Den.) sp. were clearly diurnal, while An. albitarsis, Cx. chidesteri and Cx. quinquefasciatus were obtained only at sunset and at night. Although An. aquasalis, Cx. coronator, Cx. saltanensis, Cx. crybda and Cq. venezuelensis appeared to be night feeders and Ph. deanei and Ph. theobaldi diurnal, we have caught them soemtimes out of their preferential feeding periods; Ph. deanei presented a pre-crepuscular increase in its biting activity. Ae. scapularis, Ae. taeniorhynchus and Ma. titillans, were more ecletic, attacking in large numbers at sunset, keeping very active through the night, but biting during the remainder of the 24-hour period.