Collections and dissections of black flies from human subjects were carried out over a 13-month period at two onchocerciasis-endemic sites in Guatemala. Simulium ochraceum comprised 87% of all flies collected. Populations of this species peaked during the first part of the dry season in November, while populations of S. metallicum, S. callidum and S. downsi were highest during the rainy season from June-October. Diurnal patterns of host-seeking activity were most pronounced for S. ochraceum, with a sharp peak occurring between 0700 and 0900 hours each day. Parous ratio was lowest at this time (27%), and rose to a peak of 63% between 1200 and 1400 hours each day. Eighty-eight percent of all parous S. ochraceum collected during 1200-1400 hours had large dilatations in the tunica of the ovarioles resulting from recent oviposition, thus indicating that this species oviposits in the morning and immediately seeks the subsequent blood meal. Parous biting density (the product of the total number of flies biting at a given time and the corresponding parous ratio) showed two distinct diurnal peaks, one in the early morning characterized by low parous ratio and high total number of flies, and the other in the early afternoon characterized by high parous ratio and low total number of flies. Moreover, the diurnal pattern of biting density of filaria-infected S. ochraceum was similar to that of parous biting density.