Tomatoes grown under greenhouse conditions were sprayed with radiolabelled maneb and zineb to determine the extent of degradation of these fungicides to ethylenethiourea (ETU) and to study the persistence of ETU on the fruits. The total (¹⁴C) residues decreased from 0.082 mg/kg and 0.11 mg/kg at day 0 to 0.023 mg/ kg and 0.05 mg/kg at day 20, on zineb- and maneb-treated fruits, respectively. This reduction was mainly due to the rapid growth of the fruits. ETU residues on tomato fruits were found to decline with time. A sharp reduction in ETU content was observed during the first 24h after treatment, followed by a slow decline in the following 5 days. ETU content was reduced by about 80% by day 20 after the fungicide application, and the concentration of EU, the major degradation product of ETU, doubled during the same period. Studies with tomatoes fortified with (¹⁴C) ETU (0.006 mg/kg) prior to processing into tomato paste showed that 70% of the radioactivity was lost during washing of the tomatoes in water. Further losses of ETU occurred during boiling of the juice (6%) and during storage of the tomato paste for a period of 3 weeks (3%).