Most medium formulations for improving culture of entomopathogenic nematodes (EPN) based on protein sources have used enriched media like animal feed such as dried egg yolk, lactalbumin, and liver extract, among other ingredients. Most results, however, showed unstable yields and longer production time. Many of the results do not show the detailed parameters of fermentation. Soy flour, cotton seed flour, corn gluten meal, casein powder, soytone, peptone, casein hydrolysates, and lactalbumin hydrolysate as protein sources were tested to determine the source to support optimal symbiotic bacteria and nematode growth. The protein hydrolysates selected did not improve bacterial cell mass compared with the yeast extract control, but soy flour was the best, showing 75.1% recovery and producing more bacterial cell number ($1.4{ imes}10^9$/ml) than all other sources. The highest yield ($1.85{ imes}10^5$ IJs/ml), yield coefficient ($1.67{ imes}10^6$ IJs/g medium), and productivity ($1.32{ imes}10^7$ IJs/l/day) were also achieved at enriched medium with soybean protein.