ABSTRACT
The water mite Unionicola tetrafurcatus Ramadan and Aboul-Dahab, 2002, is a symbiont on the freshwater mussels, Caelatura aegyptiaca (Cailliaud) and Unio abyssinicus Martens. The two hosts harbored 91 and 88% female mites, respectively. The infection percent of the different stages of the mite was related to the host size, and was 100% all-over the year, except in August when it sharply dropped to 26.7 and 40% for the two respective host populations. The density of female mites was correlated with the host size, and the sex ratio approached 10.7female:l male and 7.4female :l male in the two host populations, respectively. The population of mite stages significantly increased with increase in the size of the two hosts. In other words, the number of different stages increased linearly or proportional to the host body length. The most common number of mite stages was restricted between 3.5 and 4.9 cm and between 3 and 4.4 em in the two respective hosts.