The effect of row covers on watermelon (Citrullus vulgaris) grown on plastic mulch is well documented. However, row covers have not been adequately evaluated under Mexican growing conditions. Watermelon plants were grown on plastic mulch alone or with row covers to study their effect on photosynthesis, and early and total yields. Treatments were clear plastic mulch (C); black plastic mulch (B); B plus a Vispore cover of polypropylene (BV); B plus a white cover of perforated polyethylene (BW); B plus a clear cover of perforated polyethylene (BC); B plus an Agribon cover of polypropylene (BA); and bare soil as the control. For plants grown in treatment C, B or B combined with row covers, plant dry weight and number of leaves 40 days after seeding (DAS) were higher than the control (P<0.05), except BC which was comparable to the control. Total yields of treatments C, B, BV, BW and BA differed (P<0.05) from BC and the control. Total yields increased by 46.1, 43.2, 35.7, 41.6, 13.3, 15.4 and 35.8 t/ha for the C, B, BV, BW, BC and BA treatments, respectively, over the control, which yielded 27.1 t/ha. Total yield was highest for the C and the B treatment suggesting that there was no need for additional covers. The number of leaves per plant or dry weight per plant better explains the changes in yield than net photosynthesis rate. Stomatal conductance was explained in part by changes in photosynthesis.