An ectoperitrophic association of microorganisms with the fairy shrimp Branchinella spinosa was investigated by standard plate counts and scanning electron microscopy. Yeasts were found to form a monolayer structure attached to the inner surface of the peritrophic matrix suggesting a strong symbiotic association. The bacterial population consisted mostly of members of the genus Serratia, concentrated in the hindgut, and exhibiting chitinolytic activity.