Abstract: Intercellular and extracellular organic acid concentrations of industrial culture of 5. avermitills were analyzed in several typical batches of avermectin fermentation. Experimental results showed that organic acids accumulation in low production batches and high ones were totally different. In low production batch, intercellular pyruvic acid concentrations reached 31.6 mg/L at early stage of process, then de- clined to less than 5 mg/L, while excretion of pyruvic acid could be observed after 80 h of cultivation. Great amount of acetic acid, citric acid and succinic acid were excreated. The ratio of intracellular and extracellular citric acid concentration was even lower than 0.1 in early stage. This phenomenon indicated that the growth phase was extended and was even overlapped a lot with production phase, which was an important reason for the final low potency.