Common descriptions of articulatory requirements for production of the alveolar trill /r/ mainly focus on describing the configuration of the anterior portions of the tongue, while in contrast, the more posterior parts receive limited attention. Understanding how the posterior portions of the tongue move is vital for understanding speech motor control, especially with regard to speech development. This study used ultrasound imaging of /r/ productions by adult native speakers of Spanish to characterize the contribution of the posterior portion of the tongue to effective production of the sound. The results show that beyond raising the anterior portion of the tongue for trilling, Spanish speakers also retract the back part of their tongue (i.e., the tongue root). This movement resembles that seen for the production of the rhotic English approximant ([voiced alveolar approximant]). Clinical implications are discussed.