Surface properties of Cryptosporidium parvum oocysts were investigated by using electrophoretic mobility and hydrophobicity measurements. Oocysts purified from calf feces by several sucrose flotation steps and deionized water (DI) washes (DIS method) had an electrophoretic mobility (neutral surface charge) near 0.0 m² V¹ s¹ over a pH range of 2 to 10. The mean electrophoretic mobility of oocysts stored in DI containing a mixture of antibiotics had a lower standard deviation ( = 0.36) than that of oocysts stored in DI without antibiotics ( = 0.53); their electrophoretic mobility remained unchanged up to 121 days after collection. The electrophoretic mobility of oocysts purified on a cold Percoll-sucrose gradient after the feces was defatted with ethyl acetate (EAPS method) varied linearly with pH from 0.0 m² V¹ s¹ at pH 2.4 to 3.2 × 10⁸ m² V¹ s¹ at pH 10 ( = 0.52), thus displaying the negative surface charge at neutral pH observed by other researchers. The hydrophobicity of oocysts and two types of polystyrene beads was measured as a function of ionic strength by adhesion to polystyrene. Oocysts were purified by the DIS method. The ionic strength of the suspending solution was varied from 0 to 95 mmol liter¹. Two-week-old oocysts exhibited strong adhesion (~85%) at ionic strengths of 0 to 10 mmol liter¹ and moderate adhesion (~20%) at ionic strengths of 20 to 95 mmol liter¹. Two-month-old oocysts exhibited high adhesion (~60 to 80%) at all ionic strengths. These results show that adhesion properties governed by the electrophoretic mobility of purified C. parvum oocysts can be altered by the method of purification and that hydrophobicity can change as oocysts age.