The phototactic behaviour of adults of the Sudanese fairy shrimp Streptocephalus probiscideus was studied under laboratory conditions. Males were less negatively phototactic than females. This was also evident when colour filters were used. females only became little less negatively phototactic under yellow light, whereas males showed a strong positively phototactic response. The response to the positioning of a yellow filter was stronger than to the use of a red or blue filter for both sexes. The laboratory findings were compared with casual field observations on Streptocaphalus torvicornis that indicate differential vertical distribution between the sexes and a nocturnal vertical migration. Migratory behaviour with ascent starting at dusk is also predicted for S. proboscideus. This behaviour may reduce common stress factors in desert pools such as photodamage, visual predation pressure, and high surface temperatures. © 1995 Kluwer Academic Publishers.