Experiments were conducted to estimate the susceptibility of four watermelon cultivars (Aswan, Daytona, Molokai and Giza-I) to infestation with Tetranychus urticae Koch and its population fluctuation during the two successive seasons, summer 2009 and early summer 20 I 0 at Qalubia Governorate. Aswan cultivar was the most highly susceptible recording average of 42.86 and 57.59 mite moving stages/leaf during the two successive seasons, respectively, followed by the moderately infested cultivars (Daytona) recording 28.10 and 39.75 mite individuals during the two seasons, respectively. The lowest infestation was recorded on Molokai and Giza-1 cultivars, being 19.18 & 25.90 for the former and 18.10 & 25.94 for the latter cultivar during the two successive seasons, respectively. Susceptibility of watermelon cultivars to infestation with T. urticae may be affected by plant leaf morphological structure and its chemical contents. The number of trichomes/cm² leaf averaged 647.78, 744.44, 1111.11 and 1296.67 for Aswan, Daytona, Molokai and Giza-1 respectively, the less number of trichomes the more mite infestation. Positive relationships occurred between mite infestation levels and total amino acids, free amino acids, nitrogen and total soluble sugars in watermelon cultivars, while negative relationship found with tannins and nearly shown with total phenolic compounds, total flavonoids and total carotenes. Mite populations reached its peak during July and May in the first and second seasons, respectively, and infestation was lower in the first season than in the second, which might be due to early plantation.