Abstract: Human activities have profoundly affected the environment and population structures of fishes in Yangtze lakes. In order to evaluate the effects of environmental degradation on fish population structure, a comparative study on population characteristics of crucian carp Carassius auratus between a eutrophic urban lake (Yuehu Lake) and a macrophytic suburban lake (Niushan Lake), both in Wuhan, was carried out during March to May 2006. Fishes were collected and compared from the two different lakes, and the water quality was measured at the same time. Results showed that: (1) pH, conductivity, TN-N, TP-P and Chl. a of Yuehu Lake were higher than those of Niushan Lake. The numbers of total fish species and carnivorous fish species in the Yuehu Lake were fewer than those in the Niushan Lake. (2) The relative biomass of crucian carp in Yuehu Lake were higher than that in Niushan Lake, as per the multimesh gillnet catch per unit effort (CPUE) in Yuehu lake was higher. (3) Length distributions of crucian carp in both Niushan and Yuehu Lakes showed typical distributions when tested by kolmogorov-Smirnov, with a length range 63-271 mm (peak at 150 mm) and 72-191 mm (peak at 120 mm), respectively. (4) The crucian carp in Niushan Lake had a more complex age structure with six age groups, while Yuehu Lake had only three, with age 1 representing the largest proportion of 89%. (5) The relationship between weight (W) and total length (L) has been represented by the equation W= aLb, where W=1*10-L3.06 (R2=0.9893) in the Niushan Lake and W=6?0-6L3.22 (R2=0.9649) in the Yuehu Lake. Analysis of covariance demonstrated that the length-weight relationship of crucian carp are significantly different between the two lakes (P0.01). Meanwhile, the crucian carp in Yuehu Lake showed a growth advantage compared to those in Niushan Lake for fish condition factors of males and females from all of the three length groups in Yuehu Lake were all significantly greater than those in Niushan Lake. (6) As lengths of crucian carps in both lakes were increasing, the relative proportion of males declined; this change for male crucian carp in Yuehu Lake was more acute than in Niushan Lake.