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Deviation from formal null models and the comparisons between age classes are amongst the most direct methods of demonstrating ongoing natural selection. We have sampled 207 males from a population of the grasshopper Leptysma argentina at the onset and the end of the adult life span for this species; generations are synchronical and do not overlap. The karyotypic frequencies for a fusion (F) and a supernumerary segment (s1) were examined. The young male sample was heterogeneous and did not fit to the Hardy-Weinberg proportions, whereas the aged male sample did not deviate from H-W expectations in both analyses. Likewise, the young male sample showed fitness heterosis for the s1 carriers that are fusion homozygotes. We interpreted the significant results of karyotype frequency in both samples as the result of survival selection. Evidently natural selection acts on male adult viability as well as on other fitness components prior to adult survival, thus changing karyotype frequencies between generations. Likewise, we also detected a strong interaction between polymorphisms, whereas heterosis for s1 heterozygotes might be contributing to the maintenance of the s1 polymorphism. The actual target of selection remains unidentified.