Chromosome observations of triatomine bugs known as medically important vectors of American tripanosomiasis (Chagas' disease) were attempted. We demonstrated by the C-band technique that brazilian strain of Triatoma infestans includes at least three cytotypes in terms of constitutive heterochromatin polymorphism. We found cytological evidence that there is a significant difference in cytotype frequencies betweenobserved values and expected ones from Hardy-Weinberg principle. This evidence suggests a genetic regulation or an incompatibility mechanism between cytotypes, which will serve as hopeful cytological markers for vector control