This paper compares filters for US Environmental Protection Agency's Method 1622 (isolation and detection of oocysts) analysis of eleven environmental field samples from diverse locations throughout the US. Also, analyst proficiency was validated for C. parvum analysis by performing Initial and Ongoing Precision and Recovery experiments and Method Blanks as directed in Method 1622. Replicate 10-liter field samples, unspiked and laboratory spiked with approximately 100 to 250 oocysts, were tested to evaluate the matrix effects from the environmental waters. Two analysts microscopically examined the samples to ensure validity of the counts; oocysts identified in the unspiked field samples were confirmed by DAPI stain and Differential Interference Contrast microscopy. Results demonstrate that C. parvum oocysts can be recovered from environmental waters using Method 1622 but recoveries are lower and more variable than from reagent grade water. Also, the concentration of C. perfringens and E. coli tended to be much higher than the concentrations of C. parvum oocysts in these environmental waters. Includes 20 references, tables.