The cotton mealybug, Phenacoccus solenopsis Tinsley is an important pest that infested many vegetable crops. Aenasius arizonensis (Girault) was found associated with this pest. In this study, the effect of different densities of P. solenopsis on the parasitism ratio of A. arizonensis females and calculation of the parasitoid to host ratios mathematically (PHR) was carried out under laboratory constant conditions of 25±1˚C, 70±5% RH and 12:12 (L:D) hrs. The densities of 10, 20, 30, and 40 individuals of the third nymphal instar were tested: 10, 20, 30, and 40 nymphs. The obtained results showed that there were significant differences between the percentage of parasitism by mated and unmated A. arizonensis females and the density of the third nymphal instar of the cotton mealybug whereas LSD values were 10.75 and 6.66, respectively. The highest parasitism percentage was recorded with the lowest value of host density in all the treatments. Parasitoid to host ratio (PHR) was calculated mathematically at 50 % (PHR50) equaled 21.55 individuals and PHR25 equaled35.92 individuals for mated females, but PHR25 was 22.29 individuals for unmated females. The obtained results are useful in developing (IPM) programs against the previously mentioned pest through the effect of host density on the associated parasitoid efficacy for choosing the best time to release parasitoid. The present study suggests the possibility of utilization of A. arizonensis as an ideal biocontrol agent to reduce P. solenopsis populations.