Suppurative corneal ulcers due to filamentous fungi are a serious and intractable problem in many tropical developing countries. In vitro studies and a small pilot study have shown that chlorhexidine digluconate is effective. The aim was to establish the optimum concentration which would be appropriate to use in a larger randomized clinical trial. A masked randomized clinical trial of three concentrations of chlorhexidine digluconate compared with natamycin 5% was carried out in consecutive patients with established corneal ulcers shown by microscopy to contain fungal hyphae and later proven to be culture positive. Topical treatments were applied 1/2-hourly to 2-hourly for up to 5 days, with reduced frequency thereafter, and all patients were re-assessed at 21 days.Of 60 patients entered in the trial, 2 were lost to follow-up, and 12 were classified as 'severe' with little prospect of recovery. At 5 days the response was related to the concentration of chlorhexidine digluconate , with 0.2% giving the best results. Compared with the response to natamycin as the referent, the relative efficacy was 1.17 with chlorhexidine digluconate 0.05%, 1.43 with 0.1%, and 2.00 with 0.2%. The superiority of 0.2% chlorhexidine digluconate over natamycin was statistically significant (relative efficacy 2.20, p = 0.043) in patients not having had prior antifungal treatment.This preliminary study justifies further trials of chlorhexidine digluconate as a primary treatment for fungal corneal ulcers in circumstances where specific antifungal agents are not available.