Because egg production in Tabanus atratus, T. catenatus, T. lineola, T. quinquevittatus, and T. similis is not known in southwestern Quebec, potential fecundity and body size were evaluated respectively from the analysis of ovariole number and wing length in field-collected females. Ovariole number appears to be proportional to the third power of wing length, and fecundity indices can be derived from that relationship. Significant differences in body size and potential fecundity of the species suggest that there are different reproductive strategies within the genus Tabanus. Body size and potential fecundity varied significantly in both sampling years in T. lineola but not in T. quinquevittatus or T. similis. These results may prove useful in future studies of population dynamics or phylogeny.