Mental representations are complex and highly organized cognitive structures that experienced individuals develop over time in a given domain. Essentially, they enable experts to process information more efficiently, make quicker and more accurate decisions, and plan and execute actions more effectively than novices. According to the General Theory of Expertise proposed by Anders Ericsson, one of the abilities that characterizes an expert is the effective use of mental representations during their performances. Idiomatic musical improvisation is a creative and spontaneous process of generating new music that occurs strictly within the established conventions, vocabulary, and grammar of a particular musical style or genre. It is based on the deep internalization and utilization of a repertoire of patterns, schemas, and performative practices specific to that idiom. It, therefore, involves the use of mental representations, which can be refined over years of practice. The aim of this research was to compare the use of mental representations between expert and novice musicians in an Idiomatic improvisation task in a Brazilian musical repertoire. An experiment was conducted with the participation of six musicians divided into two groups: three experts and three novices. Each group consisted of a pianist, a guitarist, and a saxophonist. The procedure involved the application of a retrospective think-aloud protocol on a musical improvisation task, in which each participant recorded a performance containing theme exposition and improvisation over an audio track that included a backing track of the piece "Berimbau" by Baden Powell and Vinicius de Moraes. Then, they watched their own performance and reported verbally their mental representations throughout it. All verbalizations were transcribed and subjected to a content analysis. Results indicate that the primary concern of novice improvisers during the task was related to the execution of technical aspects of the instrument (e.g., scale/chord relationship), while expert improvisers articulated the structure of the improvisation based on the musical genre and strategies that could undergo modifications during the performance itself. It is concluded that the mental representations of experienced musicians were produced more easily and in a more complex way than those of novice musicians throughout their respective improvisations. This study contributes to important implications to instrument teaching, enabling the creation of teaching strategies aimed at improving idiomatic improvisation, based on the progressive refinement of mental representations across various styles and genres of Brazilian music.