Fusarium oxysporum f. sp. vasinfectum (FOV) race 4 is an aggressive wilt pathogen of cotton that is also capable of causing seedling mortality. FOV race 4, first identified in California in 2001, has been a recurring and expanding threat to California cotton. The goal of this research was to genotype and phenotype current field populations of FOV race 4 in California. A total of 138 isolates were collected from seven commercial or grower fields in the San Joaquin Valley during the 2017 and 2018 growing season. Isolates were genotyped using FOV race 4 specific primers, and DNA sequencing of the translation elongation factor gene. Previously only one haplotype of FOV race 4 was identified in California, however our results identified two unique haplotypes. Sixteen representative isolates were evaluated for wilt and seedling disease capabilities using a root dip inoculation and rolled towel assay, respectively. Plant material for both assays included the FOV race 4-moderately resistant Upland cultivar FM-2334 and susceptible Pima cultivar PHY-830. For the root dip assay, plants were evaluated at six weeks for foliar and vascular discoloration symptoms. For the rolled towel assay, seedlings were rated at ten days using a disease severity index and ordinal rating scale. There was a significant difference for isolate and variety (P<0.0001) for all data collected in both assays. All isolates were able to produce wilt and seedling symptoms on cotton.