Sclerotinia sclerotiorum was isolated from lettuce (Lactuca sativa) grown in Alaska. Sections of cabbage (Brassica oleracea var. capitata) and lettuce were inoculated with isolates A and B. Sclerotia of S. sclerotiorum were counted and weighed after the host tissue decomposed. Isolate A produced a mean of 42 sclerotia weighing 1.9 g on 100 g of cabbage 'Balbro', which was twice the number of sclerotia and triple the biomass produced by isolate B. On three cultivars of lettuce, isolate A had a greater mass and a similar number of sclerotia compared with isolate B. Carrot (Daucus carota subsp. sativus) and celery (Apium graveolens) were also inoculated to measure production of sclerotia. Differences between isolates of S. sclerotiorum were less pronounced on carrot and celery, but the difference between hosts was significant. The mean mass of sclerotia for isolate A was 73 mg on carrot and 18 mg on celery. Isolates A and B were distinct in the rDNA intergenic spacer sequences. Isolate A had identical sequences to those reported for some isolates from United States and Canada, whereas isolate B matched different isolates from United States, Canada, and New Zealand. Both isolates belong to a relatively recently evolved group exhibiting significant associations between genotype and both geography and host species. In agricultural fields, one infection of S. sclerotiorum on lettuce or cabbage can produce hundreds of new sclerotia that may affect levels of disease in future years. Cultural control can include minimizing the amount of sclerotia on crop residue by disking soon after harvest.