Contrasting colors of plastic mulch (black and white over black) were used to modify the rate at which heat units (HU) were accumulated in four different microclimates surrounding watermelon plants during 1996 at the Texas Agricultural Experiment Station-Stephenville. Daily maximum and minimum temperatures from 25 Mar. through 4 Aug. were recorded for air 10 cm above the mulch surface, at the mulch surface, at the soil surface under mulch, and 10 cm below the soil surface under mulch. Accumulated HU were significantly higher for white than for black mulch during two of the four periods monitored; however, the reverse was true for all other points of measurements at all times. Daily mean soil surface heat gain was 3.29 HU higher under black than under white mulch in early season, 6.21 higher in late April and early May, 5.19 higher in late May and June, and 4.19 higher in late June through July. Values for soil at 10-cm depth paralleled those for soil surface.