Abstract: In late April of 2002 and 2003,we collected 423 Reevese s butterfly lizards (Leiolepis reevesii) from a population in Ledong,Hainan,to study sexual dimorphism and reproductive output.The minimal reproductive female in our sample was 89.0 mm snout-vent length (SVL).Individuals larger than this size were arbitrarily judged as adults.The lizard is sexually dimorphic,with males being larger in both body size and head size.The rate (slope) at which head size (both head length and head width) increased with increasing SVL was greater in male adults than in female adults,whereas it did not differ between male and female juveniles.A two-way ANOVA,with sex and age (adult and juvenile) as the factors,on the residuals of the regressions of head length and head width on SVL showed that heads were larger in males than in females,and that juveniles had larger heads relative to SVL than did adults.Females maintained in the laboratory laid eggs between late May and mid-July,and they had the potential to lay multiple clutches per year.The coefficient of variation was 0.18 for clutch size and 0.13 for egg mass,indicating that clutch size was more variable than egg mass.The clutch size,clutch mass and egg mass were all independent of maternal size (SVL).The egg mass was not negatively correlated with relative fecundity,suggesting that the trade-off between size and number of eggs was absent in the lizard.The relative clutch mass was negatively correlated with maternal size,indicating that the smaller reproductive female had the relatively greater reproductive output.Given that a female s reproduction retards her growth,the relatively greater reproductive output in smaller females at least partly explains why females have the smaller adult size than do males.