The paper reports results of single-species tests with the mosquito microsporidian Nosema algerae and the orthopteran microsporidian N. locustae on nontarget aquatic organisms. Organisms tested were the freshwater grass shrimp (Palaemonetes kadiakensis), the estuarine grass shrimp (P. pugio), the marine rotifer (Brachionus plicatilis), and the inland silverside (Menidia beryllina). These organisms were exposed by intrahemocoelic injection, gavage, or ingestion. Infections did not develop in either the freshwater grass shrimp or estuarine grass shrimp that were gavaged with N. algerae spores. However, infections did develop in both species of grass shrimp after intrahemocoelic injections with N. algerae spores. Infected tissues included the gills, antennal gland, eyes, skeletal muscle, heart, and gonads. Proof of infection was demonstrated ultrastructurally by the presence of mature spores and developmental stages in infected tissues.