Numerous unpublished studies indicate that Euplotes crassus is resistant to extremely high concentrations of commonly used selective agents when grow in artificial seawater. We have found that when E. crassus is grown in 1 part artificial seawater + 9 parts 0.3 M glucose, they are much more sensitive to a number of selective agents. For example, 1080 ug\/ml paromomycin in artificial seawater only inhibited growth, as little as 120 ug\/ml was effective in killing cells when grown with 0.3 M glucose. Similarly, G418 at 400 ug\/ml had little effect in seawater, but as little as 100 ug\/ml blocked growth in 0.3 M glucose. This may not prove true for all selective agents, as we saw no effect with paclitaxel up to 100 uM under either growth condition.