The period gene determines biological rhythmicity in Drosophila melanogaster. The X-linked gene is 7.4kb, containing 8 exons and 7 introns from which a 4.5kb message is translated. A striking feature of the protein encoded by per is a series of alternating threonine-glycine residues in the fifth exon. Moreover, this string of residues is polymorphic for length variation in natural populations, the most frequent variants having 17, 20 or 23 Thr-Gly pairs. In the present study, a geographic analysis of this polymorphism within North American populations was conducted, the results of which indicate significant variation of allele frequency with latitude. The use of spatial autocorrelation analysis and Mantel tests clearly show that the most common variant, encoding 17 Thr-Gly pairs, exhibits a clinal pattern in its distribution along a north-south axis. Furthermore, DNA sequence analysis of several variants has uncovered a novel new variant which encodes 22 Thr-Gly pairs whose nucleotide sequence differs from any published data. Similar statistical analysis conducted on seven allozymes for populations collected along the same transect shows that several have monotonic clinal patterns in their allele frequency distributions which also show correlation with latitude. A previous study of morphological traits on the same populations showed the existence of a non-monotonic clinal pattern. Comparison of the results observed for the molecular and morphological markers indicates that they are subject to different evolutionary forces. The results highlight the importance of comparing patterns of geographic variation using different genetic elements. / Thesis / Master of Science (MS)