The effects of microcystins (MCs) from cyanobacterial cells on the enzymatic activities of acid and alkaline phosphatases (ACP and ALP) from liver, kidney and gill tissues, and the histopathological changes in freshwater Tilapia fish (Oreochromis sp.) were investigated under laboratory conditions. Fish were exposed to cyanobacterial cells (60.0 microg MC-LR/fish per day) through their diet at different exposure times (14 and 21 days). The cells were fed to the fish in two types of oral administration: mixed with a commercial fish food or crushed into a commercial fish food so that the toxins were released. ACP and ALP activities changed in response to MCs in a time-dependent manner, and these changes were more prominent in liver and kidney. The way the MCs were administered had no influence on the biochemical parameters. Similarly, the most severe histopathological changes were observed in the same two organs, although the gills and intestines were also affected. The parenchymal architecture of the liver was dissolved, and round hepatocytes with the appearance of pyknotic nuclei were detected. Kidney lesions consisted of the dilation of Bowman's space and necrotic epithelial cells with pyknotic nuclei in the tubules. These findings suggest that low and repeated doses of MC-LR from cyanobacterial cells induce toxicity in tilapia fish although no adverse effects were detected.