Seasonal and daily activity patterns of human-biting mosquitoes were studied in the lower delta of the Paraná River from March 2003 to February 2004. Monthly captures at four daytime intervals using human volunteers collected 1,289 mosquitoes belonging to 14 species and six genera, with the most frequently captured being Ochlerotatus crinifer (49%), Psorophora ferox (36%), Ochlerotatus serratus (5%), and Isostomyia paranensis (3%). Oc. crinifer was collected during the four seasons and showed higher values in Summer and Autumn. Ps. ferox and Oc. serratus were not present in Winter and the highest values were recorded in Summer. Monthly captures of Ps. ferox and Oc. serratus were positively associated with temperature. With regard to daily activity patterns, Oc. crinifer, Oc. serratus, and Ps. ferox were captured during the four collection intervals with nocturnal captures concentrated during the warm months. The daily activity patterns of these species changed throughout the seasons.