Treatment of a small stream with Neomesomermis flumenalis preparasites resulted in black fly larval infection rates of up to 71.4%. The preparasite density and black fly infection rate decreased markedly and progressively below the point of introduction indicating poor preparasite dispersion. Simulium venustum larvae were more readily parasitized than S. vittatum. A substrate density of 0.7 preparasites/cm2 was required to achieve a PR50 (parasitism rate) for S. venustum. By extrapolating beyond the parasitism rate achieved, it was calculated that 10 preparasites/cm2 would be required for a PR90. Maintenance of these high preparasite densities in a typical black fly producing stream (e.g. 2.5 meters wide) would require application of approximately 3.6 x 10 preparasites per mile of stream. Cost for production of such large quantities of preparasites is at present too great even for intermediate stage field trials.