Of 176 black fly females collected resting at the edge of El Rosario River, 77% were caught in rocky shelters, whereas the others were captured on damp stretches of river bank. Simulium ochraceum predominated in the catches (74%), followed by Simulium metallicum (21%), and Simulium callidum (5%). Percentages of females without blood in their gut were 70, 85, and 67%, respectively. Of 55 S. ochraceum females that had recently oviposited (sacculate), 29 had the remains of a blood meal in the gut. Of 64 nulliparous females, 48 exhibited early oogenesis (Christopher's stage I-II) indicating recent emergence. At Las Golondrinas village, 36% (71) of 199 specimens were S. ochraceum, of these 85% had no blood in the midgut. Five females of S. ochraceum at this locality had red blood and 6 had black blood in their gut. Of 43 parous and 28 nulliparous females, 36 and 25 had ovaries at stages I-II, respectively. Most females of S. metallicum had no blood in their gut (92%). Nullipars were abundant (91) and stage N (24 h old) was observed in 66 of them.