This document provides a horizontal method for enumeration of microorganisms that are able to grow and form colonies in a solid medium after anaerobic incubation at 30 °C. The method is applicable to microbial plant biostimulants, except those composed of aerobic bacteria, for verifying that the concentration of anaerobes does not exceed the respective limits outlined in the EU Fertilizers Regulation [1]. This method does not apply to the microbiological monitoring of the environment in which microbial plant biostimulants are manufactured. No information about potential human pathogens can be inferred from anaerobic plate counts.