Abstract Phosphomonoesterase activity was determined for a 115,000g pellet and soluble fractions resulting from a subcellular fractioning of a homogenate of larval Boophilus microplus. Both fractions showed maximum phosphatase activity at pH 5.5 and 10. Acid phosphatase (EC 126.96.36.199) activity was found to be greatest in the soluble fraction. When the reaction rate was plotted against homogenate concentration, the soluble acid phosphatase deviated from the linear relationship. For both fractions different thermostability patterns were obtained, inactlvation beginning for the alkaline phosphatase (EC 188.8.131.52) at 45–55 C. When the effect of substrate concentration on activity was studied, deviations from the typical hyperbolic behavior were observed. Homogenization of larvae with 5 m m EDTA buffer failed to yield a low-speed pellet with high alkaline phosphatase activity, as it is expected if absorptive structures sediment. Moreover, total alkaline phosphatase activity recovered by this method is significantly lower than activity recovered when homogenization is carried out without EDTA. Alternately, homogenization with 10 m M Tris buffer and 0.25 M sucrose gave 27,000g and 115,000g fractions with high phosphatase activity when fractioned by centrifugation. Alkaline treatment of the 115,000g fraction with 10 m M Tris buffer, pH 7.8, failed to separate endoplasmic reticulum contaminants without loss of phosphatase activity. When the 115,000g fraction was centrifuged in a sucrose density gradient, two activity peaks, coincident for both acid and alkaline phosphatases, were obtained. Antigenic analysis showed the existence of similar antigenic determinants in both peaks “immunologically” presented in different ways.