The degree of reproductive contact between the populations of two intergrading forms of Myrmica rubra L. (now designated as the subspecies macrogyna and microgyna) is considered. Like matings are more frequent than unlike, but no good evidence of selection by the individuals concerned could be obtained. Rather it appears to be due to circumstances created by differences in the space-time distribution of the populations. Such differences arise directly and indirectly from the reproductive differences between the subspecies which adapt them severally to seral (macrogyna) and biotic plagioclimax (microgyna) vegetation respectively. Such intermediates (assumed to be hybrids) as occur are likely to be subviable for socio-ecological reasons (although physiological and genetic ones may reinforce these). It is suggested that evolution from a less specialised form must have occurred during spatial isolation under different ecological conditions.