The Santa Ana River, California, originally contained native coast rainbow trout, but repeated introductions of hatchery-reared rainbow trout and Lahontan cutthroat trout have subsequently been made. Twenty-four morphological characters of samples of native coast rainbow trout, hatchery-reared rainbow trout, and cutthroat trout, and recent Santa Ana River rainbow trout were compared using the D2 statistic to determine the extent that the native type was modified by introductions. The character mean of the recent Santa Ana sample was 10.8 D2 units away from that of the cutthroat trout sample, 7.3 units from the hatchery-reared rainbow trout sample mean, but only 3.0 units from the native rainbow trout sample mean. Pure Lahontan cutthroat trout have disappeared from the Santa Ana River and trout recently occupying this river closely resemble the native coast rainbow trout.