Investigations were conducted on the sensitivity to isometamidium chloride (Samorin) and diminazene aceturate (Berenil) of derivatives of three of the Trypanosoma congolense stocks isolated between 1978 and 1983 from Zebu cattle in the Bobo-Dioulasso region of Burkina Faso. Boran cattle were used in the drug-sensitivity tests and were infected using Glossina morsitans centralis. The results showed that T. congolense stock IL 2466 isolated in 1978 was sensitive to the standard therapeutic dose of isometamidium chloride (0.25 mg kg-1) and of diminazene aceturate (a.i. 3.5 mg kg-1). However, T. congolense stock IL 2468 isolated in 1982 was resistant to both the prophylactic (0.5 and 1.0 mg kg-1) as well as the therapeutic doses of isometamidium chloride (up to 1.0 mg kg-1) although the sensitivity to the therapeutic dose of diminazene aceturate (3.5 mg kg-1) was not affected. The T. congolense stock IL 2856 isolated in 1983 was highly resistant to the therapeutic action of diminazene aceturate (up to 10.5 mg kg-1), as well as to the prophylactic (up to 1.0 mg kg-1) and therapeutic action of isometamidium chloride (up to 2.0 mg kg-1). The infection rates of the drug-resistant stocks of T. congolense in G.m. centralis, when goats were used as reservoir hosts, were as high (range, 22.3-56.3%) as of the drug sensitive stock (49.5%). The resistance trait in the two stocks remained stable after their cyclical development in the tsetse vectors. The rate of transmission of the drug-resistant stocks to mice by the infected tsetse was also high (mean 81.3%).