The fine structure of the compound eye of adult specimens of Squilla mantis was investigated. The eye consists of about 3600-3700 ommatidia, each containing a dioptric apparatus formed by a lamellated corneal lens and a eucone-type crystalline cone. Each of the four cone cells give rise to a cylindrical process (crystalline thread) inserted between the retinula cells and extending down to the basement membrane. Two distal pigment cells completely encompass the distal part of the crystalline cone, becoming progressively smaller and forming roundish processes. At the level of the tip of the crystalline cone they split off into small pigment-containing processes, and a central process leads down to the basement membrane. About 12-16 proximal pigment cells surround the ommatidium and extend from the tip of the crystalline cone to the basement membrane. In addition to the two types of pigment cell, three other types of pigment-containing cells were identified, one of which possibly contains - on the basis of their ultrastructure - crystals of the respiratory pigment hemocyanin. The two other pigments are found respectively on the surface of the retina (green pigment) and beneath the surface and in the intraommatidial space (white pigment). The distal part of each ommatidium consists of retinula cells of about equal size. Further proximal an 8th small retinula cell is encountered. The fused, centrally located rhabdom, is built up of the microvilli (rhabdomeres) of the 7 large retinula cells, the 8th has no microvilli. The structure of the ommatidia was also examined in relation to light-dark adapted conditions (LA-DA). In DA the crystalline cone shortens and the rhabdom becomes longer by an approximately corresponding amount and the pigments of the distal pigment cells expand more distally. The number and type of special cytoplasmic inclusions, as well as the shape and size of the so-called perirhabdomal vacuoles, seems not to be changed by light or dark-adaptation. In the 7 large retinula cells, an unusual migration of pigment granules occurs under DA conditions (the 8th does not contain pigment granules), In DA, many of granules which are distributed around the rhabdom in LA, line up in rows, like strings of pearls, along the rhabdom, forming a dense pigment coat around it. The findings are compared with those of related studies and their functional implications for the vision of Squilla are discussed.