Five strains of mice commonly used in transgenic and knockout production were compared with regard to genetic background and behavior. These strains were: C57BL/6J, C57BL/6NTac, 129P3/J (formerly 129/J), 129S6/SvEvTac (formerly 129/SvEvTac) and FVB/NTac. Genotypes for 342 microsatellite markers and performance in three behavioral tests (rotorod, open field activity and habituation, and contextual and cued fear conditioning) were determined. C57BL/6J and C57BL/ 6NTac were found to be true substrains; there were only 12 microsatellite differences between them. Given the data on the genetic background, one might predict that the two C57BL/6 substrains should be very similar behaviorally. Indeed, there were no significant behavioral differences between C57BL/6J and C57BL/6NTac. Contrary to literature reports on other 129 strains, 129S6/ SvEvTac often performed similarly to C57BL/6 strains, except that it was less active. FVB/NTac showed impaired rotorod learning and cued fear conditioning. Therefore, both 129S6/SvEvTac and C57BL/6 are recommended as background strains for targeted mutations when researchers want to evaluate their mice in any of these three behavior tests. However, any transgene on the FVB/NTac background should be transferred to B6.Habituation to the open field was analyzed using the parameters: total distance, center distance, velocity and vertical activity. Contrary to earlier studies, we found that all strains habituated to the open field in at least two of these parameters (center distance and velocity).