The Michaelis-Menten model has been widely used to estimate the richness (S) of species pools, but is largely untested. We tested whether (1) species accumulation curves follow the form predicted by the model, (2) the model gives unbiased estimates (Ŝ and ˆB, respectively) of S and of the sample size,B, needed to detect S/2 species, and (3) performance is robust to community structure. Performance varied with community structure. For model communities with species-abundance distributions based on MacArthur's broken-stick model with 100 or 1000 species, deviations from predicted accumulation curves were slight, and Ŝ and ˆB were unbiased (P ≥ 0.18). For broken-stick communities with 10 species, Ŝ and ˆB overestimated S andB by an average of 17% and 63%, respectively (P < 0.001). For model communities with species-abundance distributions based onTokeshi's(1990) random-fraction model with 10, 100, or 1000 species, deviations from predicted accumulation curves were large; on average, Ŝ underestimated S by 7-37% (P < 0.001), and (forS = 100 or 1000) ˆB underestimated B by 67-80% (P < 0.001). Vascular plant inventories (S = 42 to 99 species) also showed large deviations from predicted curves; on average, Ŝ underestimatedS by 35% (P < 0.001) and ˆB underestimatedB by 72%(P < 0.001). Because most natural communities are better described by the random-fraction than the broken-stick model, we suggest the Michaelis-Menten model will typically yield poor estimates of S. Moreover, we argue that accepted criteria for evaluating estimators of S are inadequate.