(1) Digitaria milanjiana (Rendle) Stapf, is a highly variable grass species which occurs in regions of low (450 mm) to high (1700 mm) rainfall in central and eastern Africa. (2) Studies of dormancy characteristics of seed of a range of accessions grown under uniform conditions revealed three different classes, apparently genetic in origin. Ecotypes from high rainfall regions had relatively short dormancy, ecotypes from low rainfall tropical regions had an extended dormancy which could be broken by treatment at 60 ⚬C for several months, and an ecotype from a low rainfall equatorial site had an intermediate length dormancy which was not affected by heat treatment. (3) Dormancy in all cases was broken by removal of the palea and lemma. Leaching of entire disseminules in running water had no effect and short-duration, high-temperature treatment (100 or 120 ⚬C for 15 s) had a variable and inconsistent effect. Treatment of dormant accessions at low temperatures (10 ⚬C for 9 months) prolonged dormancy. Treatment of all dormancy-class accessions at 60 ⚬C for 25 weeks had no immediate effect on viability, but resulted in a much more rapid fall in viability than treatment at 30-25 ⚬C or 10 ⚬C for 25 weeks. (4) Hybridization experiments showed that the dormancy of the low-rainfall equatorial ecotype, although due to the presence of the lemma and palea, was controlled by an interaction between embryo and maternal genotype. No such interaction was evident in low-rainfall tropical ecotypes. (5) The adaptive significance of these dormancy patterns is discussed in relation to the distribution of rainfall in the provenance sites. The extended dormancy of tropical accessions is clearly adaptive to a long dry season and the requirement for long periods of high temperature to break dormancy prevents seed from germinating following occasional dry season showers. The absence of dormancy in high rainfall equatorial ecotypes is consistent with the absence of a dry season in their region of origin.