Rates of embryogenesis and of development and growth in several nematodes are linearly related to temperature over a considerable range. On this basis, published data on the thermal time requirements are compared for a tropical and a temperate species of plant parasitic nematode Meloidogyne javanica and M. hapla respectively, the two being closely related and morphologically and biologically similar. M. hapla has a lower base temperature (Tb) and a higher thermal constant (S) than M. javanica with the relative values being almost inversed. Consequently, above their respective Tb values the slope of the relationship between rate of development and temperature was greater for the tropical species than that for the temperate species. A mathematical exploration of the relationship between Tb and S was made assuming that, over a narrow range, Tb×S was a constant. With this assumption, for any given average environmental temperature (Te) the optimum base temperature for minimum developmental duration was Te/2, and the temperature at which the duration of development was equal for the otherwise identical species was shown to be the sum of their base temperatures. The practical effect of the differences in Tb and S was to give M. hapla, the temperate species, a shorter life cycle and hence a competitive advantage at temperatures below 21ÅC and M. javanica, the tropical species, the advantage above that temperature. It is argued that a negative correlation between Tb and S is likely to be widespread, and provides a mechanism for regulating the distributions of related, competing organisms. Support for the hypothesis that the value of S tends to decrease as Tb increases is derived from data on the embryogenesis of an animal parasitic nematode Haemonchus contortus and from seed germination studies. Contrary results and exceptions are also briefly discussed. The observed interaction between Tb and S may be fundamental to many poikilothermic organisms and plants and provides an explanation for tropical species generally having higher Tb values than temperate ones. The ecological implications of different values of Tb and of S, including their relationship with organisms which are “r” or “K” strategists are briefly discussed.