A 3-yr evaluation of the biological control agentNosema locustaeCanning on grasshoppers was conducted on the National Wildlife Refuge at Maxwell, NM. Six O.1-ha plots were treated with the microsporidian in 1988; grasshopper densities were monitored in treatment and control plots through 1990. In 1988, there was a significant time-by-treatment interaction; grasshopper densities were significantly reduced for several weeks, with as much as 50% reduction by the end the season. Infections were highest 6 to 8 wk after treatment. Infection rates differed significantly across developmental stages and subfamilies. Melanoplinae were most heavily infected and suffered the greatest rate of mortality. Three additional orthopteran species were added to the host range ofN. locustae. In the treated plots, in 1988, bird densities were significantly higher, and the parasitic waspScelio opacus(Provancher) was significantly more frequent in egg pods. Spores ofN. loeustaewere present in unhatched eggs and early instar nymphs, but no spores were observed on egg pods or foam plugs. In June of 1989, density of the treated population was similar to that of the controls, but from mid-July through mid-August the population density was significantly lower in the treated plots. Based on infection and demographic data, this second-year population suppression seems to have been the result ofN.loeustae. In 1990, treated and control populations did not differ significantly, and both were reduced below the standard treatment threshold of 9.6 grasshoppers per square meter.