Natural rubber (NR) is a biopolymer whose properties depend on the molecular structure of the 1,4-cis polyisoprene chains, nonrubber constituents, environmental conditions, etc. NR has been characterized by Mooney viscosity, Wallace plasticity, nitrogen content (%N). However, these cannot effectively account for clone's differences. The aim of this work is to use extensional rheology to characterize and differentiate NR samples as for clone type and the season of the year in comparison to the traditional characterizations. Three IAC 300 series and RRIM 600 clones of Hevea brasiliensis tapped between October 2006 (Oct_06) and August 2008 (Aug_08) were investigated. IAC 329 clones showed the least susceptibility to seasonal changes, whereas RRIM 600 was the most influenced. An opposite trend between extensional viscosity (?E) and %N was established. The former was very sensitive to changes in the molecular structure of NR, being fundamental for monitoring purposes and strategic development of new rubber tree clones. POLYM. ENG. SCI., 2012. (C) 2011 Society of Plastics Engineers