The genome sizes of mitochondrial DNA from darkgrown (etiolated) shoots of several higher plants were determined by reassociation kinetics and restriction analysis. Kinetic complexities obtained from reassociation kinetics measured spectrophotometrically indicate a mitochondrial genome size of 1600 Md for muskmelon, 1000 Md for cucumber, 560 Md for zucchini squash and 220 Md for watermelon (four species in the cucurbit family), as well as 240 Md for pea and 320 Md for corn. The kinetic curves also reveal the presence (except in corn) of sequences of a few magadaltons of complexity, reiterated about 10-50 times and representing 5%- 10% of the DNA in each mitochondrial genome. Molecular weight summation of fragments resulting from digestion with restriction endonucleases Sal I and Kpn I give genome size estimates similar to those obtained from reassociation kinetics, except for muskmelon and cucumber, for which the large number of fragments of similar size limits our estimate to at least 500 Md. The number of mitochondrial genomes per diploid cell is estimated to be about 110 to 140 for muskmelon, zucchini and watermelon. We consider the possible evolutionary mechanisms by which the mitochondrial genome has grown within the cucurbit family and the possible reasons for the existance of a seven to eight-fold range in mitochondrial genome size among such closely related species.