The susceptibility of common nontarget aquatic organisms to the microsporidium Edhazardia aedis was investigated in the laboratory. Eight predacious species along with 9 scavengers and filter feeders were tested. The nontarget organisms were not susceptible to infection by E. aedis and there was no appreciable mortality. To measure the relative safety of E. aedis to nontarget organisms, a simple mathematical expression was employed where risk is defined as the product of the probability of exposure and the result of exposure (infection) expressed as P(e)P(i). In these laboratory tests, the probability of exposure was fixed at 1 (maximum challenge) and the probability of infection was determined to be 0. Therefore, the risk associated with release of E. aedis into the environment is considered to be negligible under these conditions. The true risk for nontarget organisms to E. aedis can only be determined by careful evaluation of controlled field studies in the natural habitat of the target host.