The minimum inhibitory concentration (MIC) of dimetridazole (DMZ) against Treponema hyodysenteriae (55 isolates) obtained over the period 1974-77 from individual pigs with swine dysentery from 41 herds where DMZ had been in use was determined. The MIC was less than or equal to 5.0 microgram per ml for 54 of the isolates and differences in the distribution of MICs between the annual sampling periods were not significant (P less than 0.05). There was no decrease in sensitivity of T hyodysenteriae to DMZ during the survey.