Studies on biotransformation of pyridine derivatives have revealed two alternative patterns of biodegradation. Pyridine and α-picoline are not degradedvia the hydroxylated derivatives. A reductive pathway involving succinic semialdehyde (SSA) or glutaric semialdehyde is involved in degradation of pyridine, α-picoline and possibly other alkyl pyridines. α-Picolinate, nicotinate, isonicotinate and hydroxypyridines are degradedvia hydroxypyridine derivatives. Studies on aNocardia strain capable of growth on pyridine-N-oxide (PNO), 2-hydroxypyridine and pyridine rule out the involvement of PNO or 2-hydroxypyridine in pyridine metabolism. PNO is metabolizedvia 2-hydroxypyridine while pyridine is metabolizedvia SSA. Biodegradation of isoniazid and isonicotinate (INA) appears to be more complex. INA is degraded in several organismsvia 2-hydroxy INA and citrazinic acid. However, a strain ofBacillus brevis isolated in our, laboratory metabolises isonicotinatevia SSA, and a pathway similar to that of N-methyl-INA may be operative. Several organisms degrading quinoline and isoquinoline have been isolated. Formation of characteristic pigments suggests the involvement of hydroxylated compounds as intermediates.