We examined 12 important monoterpenes found on the forest floor under Pinus thunbergii, and monitored their effect on the growth of a slime mold, Dictyostelium discoideum Ax-2. Four concentrations were tested for each compound (3.3, 0.33, 0.033, and 0.0033 $mu$L/mL). Relative growth rates were determined by comparing the cell counts of treated organisms with those from the controls. At a concentration of 3.3 $mu$L/mL, (1S)-(-)-$alpha$-pinene, (-)-menthone, (-)-camphene, (S)-(+)-carvone, and (1R)-(-)-fenchone strongly inhibited the development of this slime mold. In contrast, (+)-sabinene, (R)-(+)-limonene, and myrcene showed no inhibitory effects, even at the highest concentration tested. By comparing individual growth rates with the control during the incubation period, we could classify these monoterpenes into three groups: I., compounds that were able to inhibit Ax-2 growth at all concentrations; II., compounds that showed a strong inhibitory effect at treatments between 3.3 and 0.033 $mu$L/mL, and mild anti-microbial activity at the lowest concentration; and III., compounds that inhibited growth at higher concentrations (3.3 and 0.33 $mu$L/mL), but enhanced it at lower levels (0.033 and 0.0033 $mu$L/mL). Based on these results, we suggest that the inhibitory and enhancing effects of selected monoterpenes depend upon the concentration of the individual compound.