The effect of different treatments on crossing over between and within genes at the tip of the X chromosome of Drosophila melanogaster was studied to determine whether exchange in the two regions occurs by different mechanisms. In response to autosomal inversions, ɣ -radiation, and heat shock, crossing over of both types was altered in the same direction and to a comparable extent. This would be expected if there were only one crossover mechanism involved. There was some difference in response of interlocus and intralocus crossing over after mitomycin C injection but, since the effect on interlocus crossing over in two separate regions was not consistent, the significance of this result is questionable. Although double crossing over involving the two interlocus regions was very rare doubles involving an inter and an intralocus region were recovered. The association of exchanges within a gene with a crossover between genes might indicate that there are two noninterfering mechanisms, or that multiple exchanges occur in short effectively paired regions. A further experiment designed to detect switch regions is outlined. / Science, Faculty of / Zoology, Department of / Graduate