We conducted studies to determine if the xenoestrogens Surflan™ and its active ingredient oryzalin, affect indices of reproductive fitness in medaka (Oryzias latipes). Oryzalin (0.5, 0.25mg/l) or Surflan™ (2.0μl/l) and oryzalin (0.5mg/l) significantly increased the mean number of non-fertilized eggs produced by treated females paired with untreated males, or by untreated females paired with treated males. Oryzalin (1.0, 0.5, 0.25mg/l) and Surflan™ (3.8, 2.0, 1.0μl/l) significantly affected the time to hatch of eggs from treated females paired with untreated males, and from untreated females paired with treated males. Surflan™ (3.8, 2.0, 1.0μl/l) induced intersex lesions in 80–100% of males. Oryzalin-exposed males exhibited a significant increase in the incidence of necrotic spermatids and necrotic spermatogonia, while oryzalin-exposed females had significantly fewer immature oocytes and an increase in the occurrence of hyperplastic ovaries. Our results indicate that Surflan™ and oryzalin affect both reproduction and gonadal histology in male and female medaka.