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The desert tortoise (Gopherus agassizii) was listed as a U.S. threatened species in 1990 based largely oil population declines interred from mark-recapture surveys of 2.59-km(2) (1-mi(2)) plots. Since then, several census methods have been proposed and tested, but all methods still pose logistical or statistical difficulties. We conducted computer simulations using actual tortoise location data front 2 1-mi(2) plot surveys in southern California, USA, to identify strengths and weaknesses of current sampling strategies. We considered tortoise population estimates based oil these plots as "truth" and then tested various sampling methods based oil sampling smaller plots or transect lines passing through the mile squares. Data were analyzed using Schnabel’s mark-recapture estimate and program CAPTURE. Experimental subsampling with replacement of the 1-mi(2) data using 1-km(2) and 0.25-km(2) plot boundaries produced data sets of smaller plot sizes, which we compared to estimates front the 1-mi(2) plots. We also tested distance sampling by saturating a 1-mi(2) site with computer simulated transect lines, once again evaluating bias ill density estimates. Subsampling estimates from 1-km(2) plots did not differ significantly front the estimates derived at 1-mi(2). The 0.25-km(2) subsamples significantly overestimated population sizes, chiefly because too few recaptures were made. Distance sampling simulations Were biased 80% of the time and had high coefficient of variation to density ratios. Furthermore, a prospective power analysis suggested limited ability to detect population declines as high as 50%. We concluded that poor performance and bias of both sampling procedures was driven by insufficient sample Size, suggesting that all efforts must be directed to increasing numbers found in order to produce reliable results. Our results suggest that present methods may not be capable of accurately estimating desert tortoise populations.