Ochratoxin A (OTA) is a naturally occurring mycotoxin that contaminates animal feed and human food. OTA is nephrotoxic, hepatotoxic, immunosuppressive and a potent renal carcinogen in rodents. In the present study, we evaluated the genotoxicity of OTA in L5178Y tk(+/-) (3.7.2C) mouse lymphoma cells using the microwell version of the mouse lymphoma gene mutation assay (MLA) and the comet assay modified to detect oxidative DNA damage. Cells were treated for 4 hours with 0, 5, 10, 25, 50 or 100 µM of OTA in the presence and absence of exogenous metabolic activation (S9). Benzo[a]pyrene (1 µg/mL) and 4-nitroquinoline-1-oxide (0.1 µg/mL) were used as positive control with and without S9, respectively. OTA treatment produced dose-dependent increases in cytotoxicity and tk mutant frequency, with significant increases in mutant frequency detected at concentrations ≥25 µM with and without S9. Similarly treated cells were used for the comet assay conducted with and without formamidopyrimidine-DNA glycosylase for the determination of oxidative DNA damage. OTA exposure resulted in a significant increase in both direct and oxidative DNA damage, with induction of oxidative damage being greater. The results indicate that OTA is mutagenic in mouse lymphoma assay; and that OTA-generated oxidative DNA damage is, at least partially, responsible for its mutagenicity in the assay.