Factors affecting the morphology of Streptomyces avermitilis and avermectin production in submerged cultivation, including nitrogen sources, inoculum level and DO (dissolved oxygen) tension in the broth were investigated in a 50-L bioreactor. It was found that a combination of soybean meal and yeast meal as nitrogen sources and 4.3% inoculum led to pellet formation, and the pellet morphology facilitated to maintain DO > 20% in the early stage of fermentation. With the aid of image analysis tools, area and density of pellets in different batches were calculated. Results show that higher dissolved oxygen tension was favorable for pellet formation and avermectin production. (C) 2008 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.