Fitness of a single species is often measured in terms of the number of viable offspring produced. We explore the relationship between the number of ovarioles and the size of females in five species of Simuliidae in two páramo regions of Colombia. Individuals of seven species of female blackflies that landed on animals were collected from mules in the Ucumarí Regional Park (RPNU) and from cattle in the Chingaza Natural National Park (ChNNP). The ovarioles of the five most abundant species were dissected out and counted, and a one-factor ANOVA was performed to explore differences in the mean number of ovarioles produced by different female size categories. Simulium ignescens Roubaud and S. ignescens-like species were collected in RPNU and S. ignescens, Simulium muiscorum Bueno et al, Simulium cormonsi Wygodzinsky and Simulium pautense Coscarón & Takaoka, in ChNNP. In addition, we also analyzed the Pearson product-moment correlation between the mean number of ovarioles per blackfly and female size within species using those which more than 20 individuals were collected. Ovarioles were meroistic-polytrophic type. A multiple range test (least significant difference) indicated that the largest size group had the largest mean number of ovarioles per female. In the other size groups, there were approximately equal numbers of ovarioles per female. Pearson's correlation coefficient was separately measured and indicated a 50% correlation between female size and ovariole number in S. ignescens. Although ovariole number is probably a genetically driven feature, other environmental and physiological conditions, including infections, can influence the number of eggs.