Clausocalanid copepods dominate subtropical mesozooplankton communities. Their life cycles, however, are generally unknown. Here, we follow the seasonal population development and reproductive biology of Red Sea clausocalanids in the northern Gulf of Aqaba, between February 2002 and December 2003. Monthly vertical hauls taken with a Nansen net (200-μm mesh size) between 100 m and the surface revealed four species, including a new record (Clausocalanus minor) for the area. Ctenocalanus vanus abundances peaked during winter and spring, followed by Clausocalanus farrani peaking in spring and, additionally, in June 2003. In 2002, the abundance of Clausocalanus furcatus showed no distinct seasonal development of abundance, whereas in 2003, it reached high densities in January, July and November. Overall, C. vanus dominated the clausocalanid community during the first half of the year and C. furcatus during the second half of the year, whereas the percentage of C. farrani remained fairly stable throughout both years. The correlation between the environmental parameters (temperature and chlorophyll a concentration) and the proportion of mature gonads was significant only between C. vanus and temperature. Incubation experiments showed that egg production rates varied between 0-3.3 and 1.8-9.2 eggs female-1 day-1 in C. farrani and C. furcatus, respectively. Endoparasitism by dinoflagellates and the occurrence of intersexes were found in all three species, with C. farrani being the most affected. © The Author 2006. Published by Oxford University Press. All rights reserved.