The eyes of two species of fishlice (Argulus foliaceus and A. coregoni) were examined by light- and transmission electron microscopy under dark- and light-adapted conditions at night and during the day. The results demonstrate that with their large facets, wide crystalline cones, interommatidial angles of 13-17 degrees, and massive rhabdoms of the fused type, the eyes should possess considerable absolute sensitivity and be able to perceive moving objects subtending at least 7-9 degrees of arc. The microvilli of the rhabdoms possess a uniform diameter of around 55 nm and are aligned in two orthogonal directions, which suggests that the fishlouse eye has the capacity to distinguish the degree to which light is polarized. This ability would be useful in detecting host-fish against down-welling, flickering lights during the day. A circadian rhythm is apparently involved in controlling the position of the screening pigment granules of the retinula cells (dispersed at night, aggregated during the day), but not the much smaller granules of 0.25-0.30 m in diameter, present in the most distal screening pigment cells, which occupy the spaces between adjacent cones. We counted 8 axons per axon bundle emanating from the retinal cells of a single ommatidium and conclude that a tiny distally and asymmetrically placed rhabdomere belongs to the eighth retinula cell.