The lethal effects of the ear-tuft trait of the Araucana chicken are reported and the genetic basis of its inheritance is verified. The ear-tuft (Et) gene acts as an autosomal dominant with reduced penetrance in heterozygotes. This study gave two estimates of reduced penetrance, 4 and 14 percent. Homozygotes die during 17-19 days of incubation, although a few may hatch. Most of these die within a week, but occasionally an "escaper" will live to maturity; one such Et/Et individual was verified. Heterozygotes also experience increased embryonic mortality at about 20 or 21 days of incubation. In this study the average embryonic mortality among heterozygotes was 41.6 percent. Posthatch mortality also was significantly greater among tufted chicks than among nontufted chicks.