Previous studies have indicated that the Moreton and Torresian subspecies of the grasshopper Caledia captiva are distinguished by a series of pericentric rearrangements involving 8 of the 12 members of the genome. In addition, they show distinctive allozyme patterns with[Formula: see text]. The two taxa meet in southeast Queensland where they form a narrow hybrid zone. Experimental hybridization between the taxa reveals that the F2 generation is totally inviable and backcrosses are 53–64% inviable.A major component of inviability is correlated with the redistribution of chiasmata which occurs in F1 heterozygotes and leads to the production of novel recombinant chromosomes. However, the relative contributions of the effects of chromosomal heterozygosity versus those effects because of genic divergence could not be distinguished. The discovery that the metacentric Moreton subspecies represents the northerly limit of a continuous chromosomal cline has resolved this problem. At the southern end of the cline (Lakes Entrance), the karyotype is composed entirely of acro- and telo-centric chromosomes which are equivalent in their gross structure to the Torresian karyotype and there is no redistribution of chiasmata in (LE × TT) F1 hybrids. However, the Lakes Entrance population still retains allozymic and highly repeat DNA profiles similar to the Moreton taxon. Crosses between Moreton and Lakes Entrance, which are genically equivalent but chromosomally divergent, show 42% inviability in the F2 generation. Similarly, when the chromosomal component is removed, as in the (LE × TT) cross, F2 and backcross viabilities are improved by 46 and 21–36%, respectively.These data clearly indicate that the pericentric rearrangements which distinguish both the end points of the continuous cline and the two parapatric subspecies are involved in postmating reproductive isolation.Key words: chromosomal rearrangements, reproductive isolation, speciation, clines.