This study describes the substitution patterns of the trill found in naturalistic speech by 34 monolingual Spanish-speaking Costa Rican children between ages 3 and 5; 6. In the 843 target-words identified, a total of six different phonetic realizations of the trill were found: a trill [rho] (2% of the time), an assibilated rhotic [rho lozenge] (45% of the time), a post alveolar affricate [d(3)] (9% of the time), a voiced labio-dental fricative [pi) (17% of the time), a voiced interdental fricative [Delta] (9% of the time), and a lateral approximant [lambda] (3% of the time). Interestingly, these phonetic realizations were not used consistently, as children relied on two or more phonetic realizations, regularly within a single session and to pronounce the same word. [rho lozenge] surfaced most likely as a result of the children's input, whereas [pi] and [Delta] which are not part of the children's input, as well as the other phonetic realizations, surfaced most likely as approximations of the articulatorily very complex trill.