Caenorhabditis elegans is a free-living soil nematode that is commonly used as a biological model. Recently, much work has been done using the nematode as a toxicological model as well. Much of the work involving C. elegans has been performed in aquatic media, since it lives in the interstitial water of soil. However, testing in soil would be expected to more accurately reproduce the organism's normal environment and may take into consideration other factors not available in an aquatic test, i.e., toxicant availability effects due to sorption, various chemical interactions, etc. This study used a modification of a previous experimental protocol to determine 24h LC50 values for Cu in a Cecil series soil mixture, and examined the use of CuCl2 as a reference toxicant for soil toxicity testing with C. elegans. Three different methods of determining percent lethality were used, each dependent on how the number of worms missing after the recovery process was used in the lethality calculations. Only tests having ⩾80% worm recovery and ⩾90% control survival were used in determining the LC50s, by Probit analysis. The replicate LC 50 values generated a control chart for each method of calculating percent lethality. The coefficient of variation (CV) for each of the three methods was ⩽14%. The control charts and the protocol outlined in this study are intended to be used to assess test organism health and to monitor precision of future soil toxicity tests with C. elegans.