Recently we have described the design and operation of a miniature bioreactor system in which 4-16 fermentations can be performed (Gill et al., Biochem Eng J 39:164-176, 2008). Here we report on the use of thermal profiling techniques for parallel on-line monitoring of cell growth in these bioreactors based on the natural heat generated by microbial culture. Results show that the integrated heat profile during E. coli TOP10 pQR239 fermentations followed the same pattern as off-line optical density (OD) measurements. The maximum specific growth rates calculated from off-line OD and on-line thermal profiling data were in good agreement, at 0.66 +/- 0.04 and 0.69 +/- 0.05 h(-1) respectively. The combination of a parallel miniature bioreactor system with a non-invasive on-line technique for estimation of culture kinetic parameters provides a valuable approach for the rapid optimisation of microbial fermentation processes.