A simple Closed Aquatic Ecosystem (CAES) consisting of single-celled green algae (Chlorella pyrenoidosa, producer), a spiral snail (Bulinus australianus, consumer) and a data acquisition and control unit was flown on the Chinese Spacecraft SHENZ-HOU-II in January 2001 for 7 days. In order to study the effect of microgravity on the operation of CAES, a 1g centrifuge reference group in space, a ground 1g reference group and a ground 1g centrifuge reference group (1.4 g group) were run concurrently. Real-time data about algae biomass (calculated from transmission light intensity), temperature, light and centrifugation of the CAES were logged at minute intervals. It was found that algae biomass of both the microgravity group and the ground 1g centrifuge reference group (1.4 g) fluctuated during the experiment, but the algae biomass of the 1g centrifuge reference group in space and the ground 1g reference group increased during the experiment. The results may be attributable to influences of microgravity and 1.4 g gravity on the algae and snails metabolisms. Microgravity is the main factor to affect the operation of CAES in space and the contribution of microgravity to the effect was also estimated. These data may be valuable for the establishment of a complex CELSS in the future. (C) 2004 COSPAR. Published by Elsevier Ltd. All rights reserved.