The orientation dynamics of a lyotropic colloidal suspension of sepiolite clay under extensional flow have been explored by combined dichroism and small angle light scattering measurements. Extensional flow was applied using a four-roll mill to a thin film of sepiolite suspension (rigid rods 1 mum long and 0.010 mum in diameter). Analysis of transient extensional flow reversals revealed important characteristics of the orientation dynamics of these suspensions: (i) the existence of a critical volume fraction separating isotropic behavior, where no orientation persists after stretching, from nematic behavior, where permanent orientations persist during the relaxation phase; and (ii) in the nematic domain, a critical strain rate separates two flow regimes corresponding to a stable, so-called strong flow regime above the critical strain rate and an unstable, so-called weak flow regime below it. These experimental observations agree with the theoretical predictions of the model proposed by Marrucci and Maffettone (1989, 1990) who have examined the two-dimensional form of the simple molecular model of Hess (1976) and Doi (1981). What is new in the present case is that the colloidal suspension is a three-dimensional system, whereas previous experimental validations of the model concerned only two-dimensional rodlike polymers systems [Maffettone et al. (1996)] [Maruyama et al. (1998a, 1998b)]. (C) 2003 The Society of Rheology.