We studied the possible hybridization between N. pumilio and N. antarctica by analyzing morphological and isozyme characteristics of a high altitude population at Laguna Negra in northwestern Patagonia, Argentina. Samples collected from this population for morphological and genetic analyses were compared to pure populations of N. pumilio and N. antarctica. Pure species were statistically different from each other in leaf characteristics. A multivariate analysis indicated greater similarity between the hybrids and N. antarctica. Polymorphism at Laguna Negra for 13 loci was 61% which was higher than that for N. antarctica and N. pumilio (58 and 43%, respectively). We found diagnostic alleles of each pure species and the hybrids resulted similar to N. antarctica. These results indicate that natural hybridization between N. antarctica and N. pumilio, and introgression into N. antarctica take place in Laguna Negra.