Under the Information Collection Rule Supplemental Surveys(ICRSS), a year-long national survey of protozoa occurrence designed to supplement the information collected through the ICR, biweekly samples were analyzed for Cryptosporidium and Giardia from 87 drinking water plants using surface water as their source water. The ICRSS protozoa data were generated using Methods 1622 and 1623 which both exhibit higher recovery and fewer false positives than theICR method. Analysis of protozoa occurrence data collected under the ICRSS indicates that Giardia and Cryptosporidium were detected more frequently and at higher concentrations in flowing stream source waters than reservoir/lake source waters. Giardia wasdetected more frequently and at higher concentrations than Cryptosporidium independent of source water type. Analysis of Cryptosporidium and Giardia concentrations relative to other water quality parameters, including coliforms and turbidity,failed to reveal any correlations. Plants that were monitored under both the ICRSS and ICR exhibited higher detection rates in the ICRSS than the ICR, and the differences in detection rates increased with increasing confidence in the identity of the organisms counted by the laboratory. Includes 4 references, tables, figures.