The data collected by the Onchocerciasis Research and Control Project in Guatemala were theoretically analyzed. First, Muench's simple catalytic model was applied to the age distribution of the microfilaria-positive rate in each village to obtain the force of infection. The intensity of infection, expressed by force of infection, with Onchocerca volvulus in humans of individual villages was found to be clearly associated with the distribution and abundance of the vector, Simulium ochraceum. By comparing the values of the force of infection between males and females in a village, it can be decided whether humans were infected mainly outside the village or inside. The greater force of infection found in males was considered to be attributable to the more frequent visit of males than females to the mountainous areas where the density of S. ochraceum is higher. On the other hand, the force of infection of males was nearly equal to that of females when the S. ochraceum density was high inside the villages, suggesting the occurrence of within-village infection. Through further theoretical considerations, a mathematical model for the relation between the biting density of vectors and the microfilaria-positive rate of humans in a village was proposed, and the critical density of vectors for maintaining the endemic of onchocerciasis was estimated.