The relationship between allelic frequencies at the Adh locus and latitude of origin was studied using selected published data from various parts of the world and original observations. An overall increase of Adh-F with increasing latitude was observed but the relationship is not linear. Tropical populations are generally similar, having a low frequency of the F allele (average 15 per cent) and a smooth increase with latitude (one per cent for one degree). Between 30 and 42 degrees latitude, populations living in a Mediterranean climate in various parts of the world (Mediterranean countries, Australia's east coast and North America's west coast) are also similar, with a much higher average frequency of F (70 per cent), a steeper slope (two per cent) and a broader range of variability for a given latitude. In a restricted area (near Cordoba in southern Spain) numerous wild collected samples also showed a large variability, sometimes over a very short distance. Allelic frequencies in Mediterranean countries are thus quite unstable and it is proposed that this phenomenon be called a "Mediterranean instability". Further north, numerous samples from France were characterized by an even higher frequency of F (95 per cent) and a greater homogeneity over a broad geographic area. These observations are discussed and the need for more field studies is emphasized.