Morphological and discrete genetic differences are found between geographically isolated, allopatric, tick populations. However, we have found differences in sympatric tick populations. Notable differences were found in the body size and weight of Dermacentor andersoni collected from a single location in Montana, USA. These ticks were separated in groups consisting of big (B) and small (S) individuals. The objectives of this study were: (a) to characterize genetic diversity in B and S D. andersoni individuals, (b) to evaluate transmissibility of the character associated with body size and weight, and (c) to correlate morphological differences with biological, physiological, and behavioral characteristics. We found extensive genetic variation in 16S rDNA and ITS2 loci in B and S ticks and demonstrated genetic differentiation between B and S individuals. We further provide some support for Mendelian autosomal dominant transmission of characters associated with tick body size and weight. The results reported herein show that B ticks have a better reproductive success than S ticks and suggest partial reproductive isolation of S ticks.