The heritability of the ability to cause paralysis was examined in crosses of virulent and avirulent Dermacentor andersoni (Stiles) (Acari: Ixodidae). Virulence was assessed using hamster bioassay. Paralysis was caused by the virulent parental strain but not by the avirulent parental strain. Four crosses were made: Avirulent ♀ X Avirulent ♂ (AA cross), Virulent ♀ X Virulent ♂ (VV cross), Avirulent ♀ X Virulent ♂ (AV cross), and Virulent ♀ X Avirulent ♂ (VA cross). The proportion of females that produced fertile egg masses was similar among the crosses; however, VV females produced fewer fertile eggs. This was attributed to the long-term laboratory colonization of the virulent strain. All crosses had similar levels of larval and nymphal engorgement and adult eclosion. The ability to cause paralysis was detected in progeny of all crosses except the AA cross. Both heterogeneous crosses caused paralysis, but the virulence was slightly less than for the VV cross. Virulence was similar among the AV and VA crosses, indicating that virulent males were as likely as females to pass the trait to progeny. Time to paralysis was faster for the VV cross compared with the heterogeneous crosses.