Mathematic relationships between spore concentrations, delta-endotoxin concentrations and entomotoxicity (Tx) of Bacillus thuringiensis var. kurstaki HD-1 (Btk HD-1) preparations produced in six different media were analysed. The relationship between delta-endotoxin and spore concentration and SpTx-spore (specific Tx per 1000 spore) and spore concentration produced in the different media (starch industry wastewater (SIW) with total solids (TS) concentration of 15 g/L, SIW with TS of 30 g/L, SIW supplemented with 0.2% (w/v) colloidal chitin, SIW supplemented with 1.25% (w/v) cornstarch and 0.2% (v/v) Tween 80, secondary sludge, and semi-synthetic medium) strictly followed the Power law. Tx and delta endotoxin concentration followed the exponential relation whereas a definite relation between Tx and spore concentration could not be established. Spore and delta-endotoxin produced at the early time (12 h) during fermentation might be more toxic than those produced during latter period of fermentation irrespective of media used. Tx and delta-endotoxin concentration exhibited a semi-log linear relationship. Based on these findings, delta-endotoxin concentration can be determined rapidly to monitor the progress of the biopesticide production process. (C) 2012 Elsevier Ltd. All rights reserved.