Allozymic variation in proteins encoded by 20 loci was analyzed electrophoretically in 126 adult specimens representing 4 populations and 2 species of the desert landsnail Trochoidea, subgenus Xerocrassa, in a variable desert climatic back-ground of temperature and water factors. In addition, geographic variation in 3 morphological body variables of these snails was also studied. The results indicate that: (i) Most loci (55%) were strongly polymorphic; (ii) A large proportion of the polymorphic loci (55%) displayed fixation of alternative alleles either within or between species; (iii) Most of the variant alleles (75%) were not widespread, indicating sharp local and regional geographic differentiation; (iv) Southward progressive trends were found in genic diversity, some allele frequencies, shell banding and body characters. (v) The mean estimates of genetic indices are: not of alleles per locus, A=1.69; proportion of polymorphism per population P=0.41, and proportion of heterozygosity per individual, H=0.07; (vi) The level of P increases and that of H decreases southward; (vii) The amount of variation in different functional elasses of enzymes follows the Gillespie-Kojima and partly the Johnson hypotheses; (viii) Coefficients of genetic disfance, D, between populations are high, [Formula: see text], range 0.05-0.26. D's within species may be higher than between species. Likewise, D's from the northernmost population increase progressively southwards; (ix) Significant gametic phase disequilibria occur in several populations in both species; (x) Deviations from Hardy-Weinberg equilibria were found in several loci in some populations in both species; (xi) A statistically significant (P<0.001) amount of morphological variation of all 3 body variables occurs within and between species. Body diameter decreases with evaporation. (xii) P, H, and allozymic variation in several gene loci are significantly correlated with climatic variable, primarily related to some water factors and secondarily to temperature; (xiii) Shell banding was negatively correlated with solar radiation; and (xiv) Few correlations between allozymic and morphological variations were revealed.The pattern of genetic variation of Trochoidea (Xerocrassa) vetzenii and T. (X) erkelii suggests that (a) climatic selection plays a major role in allozymic (and morphological) population structure and differentiation; (b) variation in allozyme and visual polymorphisms may provide the genetic basis for the complex physiological adaptations of landsnails enabling them to survive in hostile, hot and dry deserts, and is therefore partly adaptive rather than neutral.