A new testicular feminization (Tfm) mutation found in the inbred strain of mice, C57BL/6JJms, was separated from the original line and established as a mutant strain. This line was named C57BL/6JJms-TfmJms. The process of discovery and breeding are described. Body weights were compared among male, normal virgin female and Tfm mice between 5 and 40 weeks of age. Tfm mice were generally intermediate in weight between male and virgin female mice. This result is different from that previously reported for the Tfm mouse strains and may be of interest in comparison between Tfm mice and their normal litter mates.