1.From the culture medium of bacteria, which utilize 5-amino-4-chloro-2-phenyl-3(2H)-pyridazinone (pyrazon) as sole source of carbon four metabolites could be isolated. These metabolites were identified as 5-a~o-4-chloro-2-(2,3-~is-dihydroxycyclohexa-4,6-~ene-1-y1)-3(2H)-pyridazinone (compound I), as 2-(5-smino-4-chloro-3-oxo-2,3-dihydro-2-pyridazino)-cis,cismuconic acid (compound 11), as 2-pyrone-6-carboxylic acid (compound 111) and as 5-amino-4-chloro-3(2H)-pyridazinone (compound IV).2. Cell-free extracts of the pyrazon-degrading bacteria were shown to metabolize 2-hydroxymuconic acid with a stoichiometric release of CO,. 3.A pathway for the bacterial degradation of 5-amino-4-chloro-2-phenyl-3(2H)-pyridazinone is proposed.5-Amino -4 -chloro -2

In cell suspensions of bacteria grown with 5-amino-4-chloro-2-phenyl-3 (2H)-pyridazinone (pyrazon) as sole source of carbon o-methyl-pvrazon and m-methyl-pyrazon were converted to the cor­ responding hydroxymethyl compounds. p-Methyl-pyrazon was not metabolized.