Output signals of a commercially available on-line laser turbidimeter exhibit fluctuations due to air and/or CO2 bubbles. A simple data processing algorithm and a personal computer software have been developed to smooth the noisy turbidity data acquired, and to utilize them for the on-line calculations of some kinetic variables involved in batch and fed-batch cultures of uniformly dispersed microorganisms. With this software, about 10(3) instantaneous turbidity data acquired over 55 s are averaged and convert it to dry cell concentration, X, every minute. Also, volume of the culture broth, V, is estimated from the averaged output data of weight loss of feed solution reservoir, W, using an electronic balance on which the reservoir is placed. Then, the computer software is used to perform linear regression analyses over the past 30 min of the total biomass, VX, the natural logarithm of the total biomass, In(VX), and the weight loss, W, in order to calculate volumetric growth rate, d(VX)/dt, specific growth rate, mu [=dln(VX)/dt] and the rate of W, dW/dt, every minute in a fed-batch culture. The software used to perform the first-order regression analyses of VX, In(VX) and W was applied to batch or fed-batch cultures of Escherichia coli on minimum synthetic or natural complex media. Sample determination coefficients of the three different variables (VX, In(VX) and W) were close to unity, indicating that the calculations are accurate. Furthermore, growth yield, Y-X/S, and specific substrate consumption rate, q(sc), were approximately estimated from the data, d(VX)/dt and dW/dt in a ’balanced’ fed-batch culture of E. coli on the minimum synthetic medium where the computer-aided substrate-feeding system automatically matches well with the cell growth.