Significant improvement in cell growth and protein production has been achieved in Sf-9 insect cell cultures using pulse additions of multicomponent nutrient feed concentrates (Bedard et at., 1994; Chan et al., 1998). The present work focuses on investigating an alternative feeding strategy wherein the nutrients are fed in a semi continuous manner. Fed batch culture experiments were carried out to compare the two different feeding strategies, pulse and semi continuous and a process developed to achieve a cell density of 5.2 x 10(7) cells/mL of Sf-9 cells in a 3.5 L bioreactor. Production of recombinant protein p-galactosidase was carried out by infecting the cells with baculovirus at a MOI of 10 at cell densities of 17 x 10(6) cells/mL. Specific productivity could be maintained at cell densities as high as 14 x 10(6) cells/mL. The results presented indicate that the feeding method can provide significant improvements in the performance with a reduction in the amount of total nutrients added. On-line monitoring of the culture using the capacitance probe showed that the capacitance probe can be used successfully to monitor the biomass and infection process even at higher cell densities. (C) 2000 John Wiley & Sons, Inc. Biotechnol Bioeng 68: 381-388, 2000.