In the present work ectophosphatase activities of three trypanosomatid parasites of plants were characterized using intact cells. Phytomonas françai, Phytomonas mcgheei, and Herpetomonas sp. hydrolyzed p-nitro-phenylphosphate at a rate of 5.40, 7.28, and 25.58 nmol Pi/mg of protein per min, respectively. Experiments using classical inhibitors of acid phosphatases such as sodium orthovanadate (NaVO₃) and sodium fluoride (NaF) showed a decrease in phosphatase activities. Lithium fluoride (LiF) and aluminum chloride (AlCl₃) were also used. Although AlCl3 had no effect, LiF was able to promote a decrease in the phosphatase activities. Interestingly, the inhibition caused by LiF was enhanced by the addition of AlCl3 during the reaction, probably due to the formation of fluoroaluminate complexes. This effect was confirmed by cytochemical analysis. In this assay, electron-dense cerium phosphate deposits were visualized on the external surface of the three parasites.