This paper presents the results of an experiment examining the acquisition of the two Spanish rhotics by American English-speaking adult second language learners. Two levels of learners are considered: fourth semester university learners, and advanced Spanish majors and minors. These two groups of learners are compared to a native speaker group. The results show that the learners acquire the Spanish alveolar tap with a high degree of accuracy, but achieve a low degree of accuracy with the Spanish alveolar trill. Most errors in the production of the tap are due to transfer of the American English rhotic, and occur primarily in the fourth semester learners. Errors in the production of the trill are more varied than those for the tap. Fourth semester learners show transfer of the American English rhotic in just over half of their non-target productions; the more advanced learners, however, overgeneralize the tap, which accounts for 78% of their non-target productions for the trill. The results of this experiment indicate a tremendous difference in rate of acquisition of the two Spanish rhotics, and also suggest a different developmental path to acquisition for each. The results also lend at least partial support to Major's Ontogeny-Phylogeny Model of second language phonological acquisition.