The ability of nanofibrils of poly(tetrafluoroethylene) (PTFE) - in situ generated during compounding - to efficiently nucleate crystal growth of the bulk polymers isotactic polypropylene (i-PP), high-density polyethylene (HDPE), polyoxymethylene (POM), polyamide 12 (PA12) and poly(ethylene therephthalate) (PET) is demonstrated. Enhanced nucleation is shown to occur already at PTFE contents as low as 0.001% w/w in i-PP and HDPE. In addition it was found that compounding with PTFE lead to improved clarity of the latter polymers. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 114: 281-287, 2009