Euschistus heros (Hemiptera: Pentatomidae) has two allopatric strains with a hybrid zone in central Brazil. Asymmetric dispersal and gene flow between these strains of E. heros have been observed, where the South strain (SS) moves more quickly to the northern regions of the country than the North strain (NS) to the southern areas. In addition, SS generally has a bigger body size and presents dark brown coloration, and NS is usually smaller in size and presents light brown coloration. Here, we studied the reproductive behavior and tested for the presence of assortative mating and reproductive barriers between the two allopatric strains of E. heros. Nonrandom mating was observed in the SS strain based on mating choice trials and the reproductive isolation indexes. SS females and males prefer to mate with their co-specific (same strain) partner, while NS insects showed no mating preference. The insect's pronotum width was positively associated with the mating choice suggesting size-assortative mating in E. heros. Reciprocal crosses between strains yielded similar reproductive outputs when compared with pure strain crosses, suggesting similar fitness of hybrid pure strains. The asymmetric gene flow in the hybridization zone that favors SS seems to be associated with the reproductive behavior of the species, which favors the typical phenotype found in the SS populations.