Selective advantage of heterokaryotypes is known to be an important factor in the dynamics of third-chromosome polymorphisms in Drosophila pseudoobscura and persimilis (Dobzhansky, 1961). In a large group of samples from diverse natural populations of D. pseudoobscura, Dobzhansky and Levene (1948) found a general trend toward heterokaryotypic superiority for fitness. More recently, Pavlovsky and Dobzhansky (1966) set up multichromosomal populations (i.e., containing more than two third-chromosome gene arrangements) of D. pseudoobscura with flies derived from a Mather, California population. Under laboratory conditions at 25 C the relative fitness of each heterokaryotype, with one possible exception, surpassed that of all homokaryotypes. Strickberger and Wills (1966) sampled a Berkeley, California population of D. pseudoobscura for 28 consecutive months. This population was polymorphic for all eight of the well-established third-chromosome gene arrangements common to the Western United States. Since each gene arrangement persisted over the entire period, it was considered probable that each conferred an important selective advantage upon some of its carriers. Polymorphism for the two most common arrangements, which fluctuated in opposite, cyclic fashion, could also have resulted from superiority of each arrangement in a different environmental niche. Several arrangements were present at low frequencies and must have occurred largely as heterokaryotypes. It appears that a general type