Partial resistance to cereal rusts is characterized by reduction in pathogen reproduction despite a susceptible infection type (Parlevliet, 1985). In some pathosystems (e.g. blast in tropical lowland rice) such resistance has proved to be durable (Bonman et al., 1992). To identify oat germplasm with partial resistance to crown rust, caused by Puccinia coronata, a multilocation screening trial was initiated in 2002. Eighty-six entries were tested at 3 locations planted in the fall of 2002 in the southern US and at 5 locations planted in the spring of 2003 in the midwestern US and Canada using at least 2 replications per site. The line MN841804 was the resistant check and cultivars Brooks and Otana were susceptible checks. The resistant check showed an average disease level of 7.5% relative to the susceptible checks across locations. One entry, the cultivar CDC Boyer, may have a useful level of partial resistance for areas less prone to the disease. In the present study it averaged 24.5% disease relative to the checks across locations. CDC Boyer and 26 other lines showing relative severity values between 3% and 25% and low variation between locations were selected for further testing in 2004. Also, adult plants of CDC Boyer, OT389 (12.1% across locations) and IA98822-2 (17.6% across locations), are undergoing greenhouse tests with compatible races of P. coronata. Further field and greenhouse experiments are planned with these and other lines to ascertain if the lower disease level in field trials is correlated with partial resistance assessed in controlled inoculations. Bonman, J.M. et al. 1992. Annual Review Phytopathology 30:507-528; Parlevliet, J.E. 1985. In: Roelfs, A.P. and W.R. Bushnell (eds.) The Cereal Rusts vol. II. New York, Academic Press, Inc. p. 501-525.