Manila clams (Venerupis philippinarum) challenged in laboratory trials via bath exposure proved to be resistant to infections with Mikrocytos mackini (protistan parasite of unknown taxonomic affiliation), while Pacific oysters (Crassostrea gigas) challenged simultaneously using identical conditions developed infections. Although M. mackini was detected by a nucleic acid pathogen specific (PCR) assay in 10-30% of the challenged V. philippinarum that were sampled soon after exposure (0-48 h, n=40), all of the subsequent V. philippinarum (n=62) sampled 9-17 weeks post-exposure tested negative for M. mackini by PCR assay. Prevalence of infection for the exposed C. gigas (n=100) during this same period ranged from 50% to 100% by PCR assay. Infection was confirmed in the oysters (58%, n=60) by a digoxigenin-labelled DNA probe designed to detect M. mackini by in situ hybridization, but M. mackini was not found in any of the exposed Manila clams (n=63) using this technique.