We evaluated the effects of water application rates (wet = T1, moist = T2 and dry = T3) and four planting densities (S1 = one row at 7.5 cm plant spacing; S2 = two rows at 7.5 cm; S3 = one row at 15 cm; and S4 = two rows at 15 cm) on growth parameters of drip irrigated onions (Allium cepa cv. Texas Grano 502) in the semi-arid southern coast of Puerto Rico. In all plots fresh bulb weight, dry bulb weight and bulb width were linearly correlated with days after transplanting. Quadratic curves described the relationships between other growth parameters (fresh plant weight, fresh total weight, plant height, number of leaves, dry plant weight and dry total weight) with days after transplanting. No relationship was found between percent of total solids and days after transplanting. The coefficient of determination (R2) varied from 0.79 to 0.94 and the regression coefficients were significant at the 5% level.