Genetic variation of morphological and phenological characters of yellow nutsedge (Cyperus esculentus L.) from California was investigated and compared with isozyme data from the same populations. The importance of collection location, individual genotype, and isozyme genotype to phenotypic characteristics was evaluated. Analyses were conducted on 20 individuals collected from each of 10 widely separated populations in California. Replicate plants were started by tubers, grown in pots buried outdoors, and measured during the period from planting through flowering. Results showed that yellow nutsedge is variable across its range in California in all measured traits, including days to sprouting and flowering, height, rachis number and length, aboveground biomass, tuber weight, number of rays, ray length, number of spikelets per ray, as well as spikelet length and width. Population (collection location) was a larger variance component than genotype (individuals over all locations). As found in isozyme analysis, relatively more variation was found among than within populations, typical of clonally reproducing species. Overall, more variation was found in quantitative traits than in isozymes. The level of variation, based on coefficients of variation for each character and population, was similar within populations and not clearly related to cropping history or climate at each site. The two most common isozyme genotypes in the collection differed in four characters: date of emergence, weight of 10 tubers, rachis number, and mean ray length, which may represent different adaptive responses to cultural practices. These results indicate that isozymes do not reflect the high level of genetic and adaptive diversity of yellow nutsedge. Factors that are likely to determine the patterns of variation in this species include breeding system (vegetative reproduction), founder effects, polyploidy, and homogeneity of the agricultural environmental.