Five polyethylene rowcover treatments (none, slitted white, slitted clear, chimney, and perforated) were combined factorially with four tomato [Lycopersicon esculentum (Mill)] cultivars (`PikRed', `Jetstar', `Supersonic B', and `Heinz 1810') in a 2-year experiment. Clear or white rowcovers more than doubled early yield in 1986, from 1.2 to 2.9 t·ha-1. This early yield advantage resulted from an advancement of flowering rather than an increase in fruit number or size. In Spring 1987, high temperature caused increased fruit abortion with all rowcovers, resulting in early yield reduction. Flower production on the first two clusters for either year was not reduced by high temperatures and was increased for `Heinz 1810'. Tomatoes under slitted, white covers, the best-yielding treatment in 1987, yielded only 72% of those without rowcovers. Sustained high temperature, ≈ 40C for 3 consecutive hours or more, occurred with all rowcovers and correlated with early yield loss.