Foliar moisture content was sampled in five eastern Canadian conifers and two hardwoods during 1962-65, and seasonal trends were estabished. These were basically similar from year to year despite weather differences. The moisture content of new conifer foliage and hardwood foliage was very high (over 300 per cent) at flushing, decreased sharply, and gradually levelled out by late summer. The moisture content of old conifer foliage fell during April, passed through a minimun (ca. 95 per cent) in May and early June, then rose gradually to a maximum (ca.115 per cent) in late summer. The average composite foliar moisture content of the five conifers rose from about 95 per cent in late May and early June to about 130 per cent in mid-August. Theoretical speculations based on the heat energy required to drive off differing amounts of foliar-moisture, as well as the results of laboratory flammability tests with single trees, suggest that the 35-point difference probably has a distinct effect on the behaviour of crowning forest fires.