Fertility in human males is negatively correlated with obesity and diabetes mellitus (DM). Whether CLA influences obesity and DM associated infertility has not been studied. Seven-week-old male obese and DM-2 mice (db/db, n=40) were randomized to either an 8.5% (w/w) fat diet of a CLA isomer (0.4%, w/w) or a control diet for 6 weeks. Lean (n=10) mice were fed a control diet. The db/db mice fed a control diet displayed increased abnormal morphology, decreased sperm concentration. They also had decreased cst and cgt gene expression, despite increased seminolipid concentration, and decreased expression of genes involved in spermatogenesis compared to their lean counterparts. CLA isomers increased sperm number and normal sperm morphology, influenced seminolipid concentration, seminolipid enzyme gene expression and significantly increased Ccna1 gene expression. Seminolipid and genes of spermatogenesis appear to factor into DM and obesity induced reproductive dysfunction. Dietary CLA isomers appear to increase functionally viable sperm and thereby improve fertility.