Attempts were made to correlate the chemical structure of pyridine and 15 pyridine derivatives with both their biodegradability by estuarine sediment microorganisms under anaerobic conditions and also with their toxicity to the marine bacterium Vibrio fischeri Beijerinck 1889 by using the Microtox bacterial assay. Among monosubstituted pyridines, comparisons of different substituents at positions C-2, C-3, or C-4 atom of the pyridine ring showed that isomers of carboxylpyridine (COOHPYR), hydroxypyridine (OHPYR), and cyanopyridine (CNPYR) were more susceptible to biotransformation than isomers of chloropyridine (ClPYR) and methylpyridine (CH3PYR) in anoxic estuarine sediment slurries under sulfidogenic conditions. Isomers with the functional group at the C-2 or C-3 atom of the pyridine ring were biotransformed faster than those with the same functional group at C-4. The only exception was 4-ClPYR, which was biotransformed within 130 days, while 2- and 3-ClPYR continued to persist in the anoxic sediment slurries. Median effect concentrations (EC50) of pyridine and pyridine derivatives were in the range of 0.027 to 49.1 mmol/L. Pyridine derivatives with -CN and -OH functional groups tended to be less toxic, while pyridine derivatives with -CH3, -Cl, and -COOH functional groups tended to be more toxic. Isomers with the substituent at C-2 were less toxic than the C-3 or C-4 isomers. There was no clear correlation between the pseudo-first-order rate constants for the microbial transformation of pyridine and its derivatives and their toxicity to the marine bacterium.