The expression of trimorphic incompatibility was investigated in a high altitude population of Oxalis compacta subsp. compacta distributed in the upper alpine belt (3100-3470 m) of the central Chile Andes. Stigma-anther reciprocity (2 populations), morph representation (7 populations), unassisted selling capacity and open-pollination fruit set (6 populations) were determined. O. compacta ssp. compacta es highly dependent on external pollinators. Although it has typical tristylous flowers, considerable seed set is possible following illegitimate intermorph and intramorph cross pollination. Open pollination fruit set varied from 9-83% and on average was lower in the higher-elevation populations. The possibility of seed set following two kinds of illegitimate cross pollination is seen as a means of enhancing reproductive output by enabling more effective use of the limited pollinator resource in the high alpine environment, where fruit set shows high inter-annual variability and there is some evidence for pollination limitation in Oxalis compacta subsp. compacta.