In order to investigate the rapid variation in the structure of Brachionus calyciflorus complex and the fitness traits of the two sibling species, the rotifers were collected once a week from Jiulian Pond during 16 July and 6 August, their COI genes were sequenced and analyzed, and their fitness parameters (average lifespan, net reproductive rate, intrinsic rate of population increase and proportion of sexual offspring) were calculated at 28 degrees C and 32 degrees C with 1.0 x 10(6), 3.0 x 10(6) and 5.0 x 10(6) cells x mL(-1) of Scenedesmus obliquus as food. In total of 35 samples, 22 haplotypes were defined, among which two distinct lineages (Lineage I and II) were revealed by phylogenetic analysis. Sequence divergence was 14.8%-15.6% between the two lineages, indicating the occurrence of two sibling species (sibling species I and II). Sibling species II occurred only in the second event of sample collection, and its relative abundance in the density of the species complex was lower (1/35). In the population of sibling species I, the clones of three shared haplotypes showed overlap, while the others showed displacement. Three-way ANOVA indicated that temperature affected the net reproductive rate, the intrinsic rate of population increase and the proportion of sexual offspring, food level affected the average lifespan, the net reproductive rate and the intrinsic rate of population increase, sibling species affected the average lifespan, the intrinsic rate of population increase and the proportion of sexual offspring. The interaction between temperature and sibling species affected the net reproductive rate and the intrinsic rate of population increase (P < 0.05), the interaction between temperature and food level affected the proportion of sexual offspring (P < 0.01), and the interaction between food level and sibling species affected the intrinsic rate of population increase of the rotifers (P < 0.05). Sibling species I had a higher intrinsic rate of population increase, a shorter average lifespan and a lower proportion of sexual offspring than sibling species II.