Abstract The genetics of T. spiralis rejection from the intestine was examined in inbred mice belonging to three phenotypic categories of expulsion: strong (NFS), intermediate (C3H), and weak (B10 · BR). Experiments used various worm doses to analyze the day of worm rejection, defined as the day at which 98% expulsion of the infectious dose occurred. The F 1 of NFS (strong) × B10 · BR (weak) was a strong responder and the F 1 of the cross C3H (intermediate) × B10 · BR (weak) was intermediate. Analysis of time of rejection among offspring of the (NFS/B10 · BR) × B10 · BR backcross showed three segregating phenotypic categories which occurred in a ratio of 1:2:1 strong:intermediate:weak. Segregation analysis of C3H/B10 · BR intercross (F 2) mice produced a ratio of 3:1, intermediate:weak. The back-cross C3H/B10 · BR to the C3H parent produced 100% intermediate offspring and the back-cross to the B10 · BR parent segregated in a 1:1 ratio of intermedate:weak. Taken together the results of both sets of crosses demonstrated that strong responsiveness was a consequence of the additive effects of two dominant genes; either gene by itself conferred intermediate responsiveness. The additive nature of these dominant genes suggested that two distinct processes each lead to the expression of worm expulsion that is phenotypically intermediate and kinetically identical.