A miniaturized calorimeter with a signal noise of less than 10 nW is presented for the first time. The outstanding signal performance was obtained due to an elaborated temperature control inside the system. With a sample volume of around 6 mu l a specific heat flow detection limit of <2 mW l(-1) was achieved, which is a prerequisite for promising microbiological applications. The performance of the new calorimeter is demonstrated by investigations of the metabolic heat production of biofilms, bacteria cultured in suspensions, and single fish embryos. (C) 2008 Elsevier B.V. All rights reserved.