Mixture of 1a(>90%) and 1b (<10%). Laboratory environmental fate and behaviour studies were generally conducted with both 14C- and 3H-labelled emamectin benzoate B1a, the major component of emamectin benzoate. Due to the similarity of the structures of emamectin bezoate B1a and emamectin benzoate B1b both components are considered to follow equivalent metabolic pathways and to show similar behaviour in all environmental compartments. This hypothesis is supported by the results of a soil metabolism study by Clark (2003) in which a similar degradation pattern was observed for emamectin benzoate B1a and emamectin benzoate B1b. It is therefore concluded that the results derived from studies conducted with emamectin benzoate B1a, the major component of emamectin benzoate, are also representative for emamectin benzoate B1b. Under aerobic conditions in viable soils emamectin benzoate is extensively degraded under the formation of multiple metabolites and carbon dioxide as the terminate product. Oxidation in the C-8a position was the favourite degradation step in most soils with 8a-OH MAB1a as major metabolite approaching levels of up to 11.1% AR and 8a-oxo MAB1a es the succeeding degradate peaking at 4.5% AR. A further major metabolite, i.e. N-nitroso MAB1a resulting from the modification of the methylamino-moiety in the C-4 position, was found at maximum levels of 15.3% AR. The metabolite MFB1a occurred a a lower level with a maximum of 6.1% AR and exceeding of 5% at 2 consecutive timepoints. In addition to these metabolites, more than 30 discrete peaks were found in soil extracts at individual levels of less than 5% AR. Five of them were identified as FAB1a, AB1a, MSB1a, aglycone, and a carboxylic acid of the cleaved macrocycle. The non-extractable residues reached their maximum level at 27.8%. There is evidence that the metabolic pathway of emamectin benzoate in microbially active soils proceeds via the following steps: hydroxylation at the C-8a position to give 8a-OH MAB1a and further oxidation at this position to 8a-oxo MAB1a. The methylamino moiety at the C-4 position is also prone to modification resulting in formylethyl- (NOA 415692), formylamino- (NOA 415693), and N-nitroso- (NOA 459720) derivatives of the parent compound. A further degradation process is the successive cleavage of saccharide rings yielding the monosaccharide (NOA 419150) and the aglycone (NOA 419153). Due to the presence of additional labile bonds, these primary and secondary residues are subject to further transformations to multiple small polar compounds, resulting from fragmentation of the central macrocyclic lactone skeleton and surrounding saccharide and spiroketal structure by microbially mediated oxidative and/or hydrolytic mechanisms. Changing the exposure conditions of emamectin benzoate on soil from darkness to light resulted in a similar metabolic pattern with the exception that the 8,9-Z isomer of MAB1a (8,9-Z-MAB1a) was formed as additional degradate. However, under irradiated conditions the rate of degradation was enhanced by a factor of 2. Changing from aerobic to anaerobic conditions resulted in a significant decrease in the efficiency of degradation of MAB1a indicating that aerobic micoorganisms can be considered the key factor in soil degradation of emamectin benzoate.