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noted above. Heterozygotes composed of the two uncommon visable alleles would be rare or absent in samples of twenty or less. This is precisely the picture seen at EST-2 in several of the Gila Basin samples involved. Such a high frequency might result if the lethal mutation coincided with a bottleneck. The majority of Gila Basin samples that are polymorphic at EST-2 have significant heterozygote deficiencies and all but one of the remainder approach significance at that locus. While many samples from outside the Gila Basin also deviate from Hardy- Weinberg expectations at EST-2, they generally have moderate numbers of heterozygotes. Moreover, there is supporting allozymic and morphological evidence for assortative mating in upper basin samples as an alternate explanation for the data configuration. Polymorphic samples from the adjacent Bill Williams system are in equilibrium at EST-2. Therefore, there may be a strong geographic component to the putative null allele possibility and it may be contained within the Gila Basin. While this hypothesis seems farfetched and improbable, as the data presently stands, it cannot be totally rejected. The post-translational (known for esterases??) hypothesis, is difficult to test without controlled experimentation, though its liklihood may be diminished if other explanations are at hand. Methodological artifacts are likewise difficult to confirm without experimentation but circumstantial evidence may be an indicator. Of the twelve Gila Basin samples cited above as having EST-2 heterozygote deficiencies, seven have short to long .strings of consecutive scores for the rarer homozygote, the longer of which, though not statistically tested, certainly vastly exceed probabilities of random occurrence. An additional one of the twelve plus four other samples, which were out of equilibrium until rescored, formerly contained similar strings but were rescored on further examination. Thusfar, interpretations of scores of the remaining seven samples have stood. Such strings must raise suspicion of common cause and the possibility of post-capture effects on samples. Because samples are run electrophetically in the same order as collected and numbered in the field, separating sampling from laboratory artifacts is difficult. It is possible that some consecutive tissue samples from among those at a given locality in this extremely hot region may have been, during extraction, allowed to remain uncooled as a group too long before freezing (in efforts to conserve dry ice, etc.), thus causing some molecular reconfiguration. It is also important to note that, if nonrandomness of scores is, indeed, evidence of such a phenomenon, then there is little indication of sampling artifacts at other localities (at most of which samples were immediately immersed in liquid nitrogen) and no reason to question the veracity of results at EST-2 for most sampling localities. There is also little evidence of such artifacts at other loci examined in the twelve samples or those from elsewhere, which lessens the probability that artifacts occurred due to laboratory procedures. 15