ABSTRACT. – Ten percent of the speckled shrimp (Metapenaeus monoceros (Fabricius, 1798)) produced in Thailand were found to be infected with the bopyrid parasite, Orbione bonnieri Nobili, 1906. Both the female and male bopyrid isopods were found to infect the host branchial chamber of the same individual. There was a positive relationship between the size of the male and female parasites, which indicates that the female and male may have developed together in the host. A positive correlation was also observed between the size of the female parasites and the carapace size of the shrimp host, which indicates that the host was infected early in life and the parasite remained inside the animal and grew with the host. Morphometric analysis indicated that infected shrimps had a significantly lower bodyweight than uninfected shrimps, which was associated with the size of the female parasite. This report provides a new record of O. bonnieri infection of M. monoceros in the Gulf of Thailand at a higher infection rate than indicated in available data regarding other bopyrid isopods