The shape of the erythrocytes can be altered by a great variety of chemical agents, such as many detergents due to their amphiphilic nature. The present study examines the effect of an anionic detergent on the shape of mature, circulating catfish red blood cells. Experimental exposure to sodium dodecyl benzene sulphonate dissolved in the water of aquaria at two sublethal concentrations (1.5 and 3 ppm), for a maximum of 15 days, induced morphological changes of normal erythrocyte shape to echinocytic form. These changes were evaluated at 5, 10 and 15 days after the start of treatment, using scanning electron microscopy. The crenated erythrocytes from animals exposed to detergent appeared either with border irregularities or undulations, without distinct spicules, or with numerous short spikes. Statistical analysis, applied to the data obtained from counting altered cells in the various experimental groups, showed no significant difference between the 1.5 ppm-treated animals at the three times and the controls, whereas a significant difference was observed between 3 ppm-treated animals compared to the controls, showing significance of action of the higher dose employed at the three times. These data suggest latent erythrocyte damage. The results are discussed in the light of the extensive bibliography concerning evaginating amphiphilic compounds and the mechanisms involved in echinocyte formation, taking into account the marked differences existing between the nucleated red blood cells of fish and those biconcave, unnucleated of mammals.