The ratio of sexually active males to fertilizable females (operational sex ratio, OSR) is the most important parameter determining the direction and intensity of sexual selection. It is widely accepted that the pronounced sexual dimorphism featured by many crustaceans is a consequence of a strongly male-biased OSR. Many studies have contributed to the understanding of function and origin of sexually selected traits, but few attempts have been made to measure the OSR. Using the rock shrimp Rhynchocinetes typus as an example, herein we propose a relatively simple method to assess the OSR of natural populations of crustaceans, as follows. The number of individuals willing to mate was estimated by the study of both the population structure (field sampling) and the readiness to mate (laboratory experiments). Field sampling showed that adult sex ratio was balanced during most seasons and that early ontogenetic male stages dominated among the males. Laboratory experiments demonstrated that females became receptive asynchronously, and that males from early ontogenetic stages showed limited readiness to mate. Based on this information, we estimated the OSR under three different scenarios. The results indicate that the OSR is highly male-biased in R. typus even in the more conservative scenario supporting the original predictions. Most male mate-competitors are early ontogenetic stages, whereas few are late stages. The results demonstrate the importance of estimating the OSR individually for shrimp categories differing primarily by sex and subsequently by individual conditions (ontogenetic stage, molt stage), which affects the probability of a potential mate to be sexually active. We suggest that this approach provides important information necessary for a better understanding of crustacean mating systems.