Two field experiments were conducted at the Experimental Research Farm, Faculty of Agriculture, Suez Canal University, Egypt during spring 2013 and 2014 using summer squash (Cucurbita pepo cv. Eskandarani) for studying the effects of PGPR strains. The objective of the first experiment was to study the efficiency of single, double and triple inoculations of Azospirillum brasilense (AC1), Bacillus subtilis (AC2) and Serratia marcescens (BM1) on summer squash fruit characters. Results showed that the double inoculations of tested PGPR strains in addition to single inoculation with Serratia marcescens produced non-significant higher fruit yield and average fruit weight compared to triple inoculation (AC1+AC2+BM1) and single inoculation with Azospirillum brasilense. However, non-inoculated control plants and plants inoculated with Bacillus subtilis showed lower fruit yield and average fruit weight. From the previous results, it concluded that the double inoculation were better than single and triple inoculations. However, the aim of the second experiment was to evaluate the efficiency of PGPR under N and P fertilization levels on plant growth, yield and nutrients uptake of summer squash as well as soil nutrient availability (available N and P). As per main effects, generally, plant growth, fruit yield, nutrients uptake and availability enhanced significantly by increasing N and P levels, except half dose of P which gave statistically equivalent values of plant fresh and dry weight as well as nitrogen uptake in shoots and fruits in addition to P in shoots compared to full dose. Also, main effect of PGPR showed that the inoculated plants gave significantly higher plant growth, fruit yield, nutrient uptake and availability compared to un-inoculated control plants. Inoculated plants with PGPR strains under full dose of N and P gave mostly highest plant growth, fruit yield, nutrients uptake and availability. This effect was at significant level in terms of fruit yield, soil nutrient availability and some nutrients uptake such as K in the shoots and N the fruits. Furthermore, PGPR significantly reduced P fertilizer application without any reduction in squash yield, especially under no and half dose of N fertilizer. Results revealed that the efficiency of PGPR strains increased by P increasing and decreased by N increasing.