MicroRNAs, a novel class of post-transcriptional gene regulators, have been demonstrated to be involved in several cellular processes regulating the expression of protein-coding genes. Here we examine murine white and brown primary cell cultures for differential expression of miRNAs. The adipogenesis-related miRNA miR-143 was highly expressed in mature white adipocytes but was low in mature brown adipocytes. Three classical "myogenic" miRNAs miR-1, miR-133a and miR-206 were absent from white adipocytes but were specifically expressed both in brown pre- and mature adipocytes, reinforcing the concept that brown adipocytes and myocytes derive from a common cell lineage that specifies energy-dissipating cells. Augmentation of adipocyte differentiation status with norepinephrine or rosiglitazone did not affect the expression of the above miRNAs, the expression levels of which were thus innately regulated. However, expression of the miRNA miR-455 was enhanced during brown adipocyte differentiation, similarly to the expression pattern of the brown adipocyte differentiation marker UCP1. In conclusion, miRNAs are differentially expressed in white and brown adipocytes and may be important in defining the common precursor cell for myocytes and brown adipocytes and thus have distinct roles in energy-storing versus energy-dissipating cells. © 2008 Wiley-Liss, Inc.