The structural organization of the midgut musculature of females of the ornithophilic species Simulium rugglesi and the mammalophilic species Simulium venustum were examined by confocal microscopy. In addition, the association between the longitudinal and circular muscles of fed S. rugglesi was investigated by transmission electron microscopy. Longitudinal and circular muscles in both species are arranged orthogonally, forming a gridlike pattern. In S. rugglesi the estimated number of longitudinal and circular muscles was 181 and 90, respectively, whereas in S. venustum there were approximately 156 longitudinal and 69 circular muscles. Longitudinal muscles are arranged in unevenly spaced sets of parallel bundles. Branching of longitudinal muscles occurs randomly along the length of the midgut in S. rugglesi and anteriorly and posteriorly to the most expanded region of the posterior midgut in S. venustum. Circular muscles exhibit less variation in bundle spacing and a more organized branching pattern. Each circular muscle bundle shares fibers with the neighboring bundles, interconnecting them along the length of the midgut. Ultrastructural observations on the midgut of S. rugglesi showed connections between longitudinal and circular muscles. A comparison of the findings from this study with those from studies on mosquitoes revealed similarities in the arrangement and structure of the midgut musculature.