At Ituzuke Air Force Base, Foster first studied judo, and later karate with Hiroko Watanabe Sensei, a Goju Ryu instructor who taught at the Itazuke Administration Annex base gym. Watanabe felt that Foster was a promising student and suggested that he study with Mamoru Yamamoto, later noted as founder of Yoshukai Karate but then still affiliated with Tsuyoshi Chitose's Chito-ryu. In 1964 Foster returned to Japan to test for second degree black belt and spent three weeks training at Yamamoto's dojo. He then returned to Japan in September 1964 to live and train in Yamamoto's dojo for approximately nineteen months.