| Brachionus rubens and B. urceolaris are normal species of freshwater rotifer in the genus Brachionus. In the aquaculture they are considered as the suitable living foods for some fish larvae because of their body volume and swimming speed. The two species of rotifers are similar in the morphology. As to the relationship of the two rotifers in taxology, different people have different opinions. In the early studies, they were classified in one species. Later, they were distinguished by the shape of the anterior spine and the eye. In this paper, the two species of rotifers were studied in the laboratory. The main results were presented as following:The effects of food concentrations on the population growth, body size, and egg size of the freshwater rotifers B. rubens and B. urceolaris were studied using population accumulative culture method. The results showed that there were very significant effects of food concentrations on the population growth rate, body volume and egg volume of the two species of rotifers. The relationship between the population growth rates of them and the food concentrations were curvilinear, and it could be described as Y=-0.004X2+0.0409X+0.4471 and Y= - 0.0024X2 + 0.0098X + 0.4385, and the maximum population growth rates of B. rubens and B. urceolaris are 0.5517d-1 and 0.4685d-1, respectively. Accordingly, the food concentrations are 5.1125×106cells · ml-1 and 2.0417×106cells · ml-1 respectively. At the food concentrations of 6.0,9.0 and 12.0×106cells · ml-1, the population growth rate of the rotifer B. rubens is larger than that of B. urceolaris. The body volume and egg volume of B. rubens tended to be smaller when the food concentrations were higher or lower than 6.0×106 cells · ml-1 and 3.0×106 cells · ml-1, respectively. As far as B. urceolaris, when food concentration is 1.0×106 cells · ml-1, the body volume was the smallest. But the egg volume was larger at the food concentration of 3.0×106 cells · ml-1. At the same food levels, the body volume and egg volume of B. urceolaris were both larger than those of B. rubens.The duration of principal developmental stages and the population growth parameters of amictic females of B. rubens and B. urceolaris were studied under the different food concentrations by individual culture method. There were no significant effects of food concentrations on the duration of every developmental stage, minimum generation time and the mean lifespan of the two species of rotifers. There was no significant difference of embryonic time (De) between these two species of rotifers. But the duration of juvenile period (Dj) of B. rubens was significant longer than that of B. urceolaris at all of the food concentrations. At the food concentration of 3.0×106 cells · ml-1, the minimum generation time (Tg) of B. rubens was also longer. The duration of reproductive period (Dr) and the mean lifespan of B. rubens were longer than those of B. urceolaris at the food concentrations of 0.75 and 3.0×106 cells · ml-1. When the food level was 0.75×106 cells · ml-1, the duration of post reproductive period (Dp) of B. rubens was longer than that of B. urceolaris. The innate capacity of increase (rm) and net reproductive rate (Ro) of B. rubens tended to be larger with the increased food concentrations. But the rm of B. urceolaris was larger at the lower food concentrations. The parameter eo of B. rubens was larger than that of B. urceolaris.The male egg-producing mictic females of the two species of rotifers were also studied at the same conditions. The De of B. rubens was significant shorter than that of B. urceolaris at the food concentration of 1.5×106 cells · ml-1. At the food levels of 3.0 and 6.0×106 cells · ml-1, the Dj of B. rubens was significant shorter than that of B. urceolaris. Except for 0.75×106 cells · ml-1, the Dr of B. rubens was longer than that of B. urceolaris on the other three food concentrations. At the food concentration of 3.0×106 cells · ml-1, the Dp of B. rubens was also longer than that of B. urceolaris. Th...
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