Pedagogical content knowledge forms the core of teachers' professional knowledge; it refers to knowledge about making subject matter accessible to students. Thus, the formation of pedagogical content knowledge constitutes a crucial issue for educational research and practice. We investigated the contributions of content knowledge and pedagogical knowledge to the formation of pedagogical content knowledge about fractions and fractional arithmetic in 6th grade mathematics in a between-participants study with 100 German preservice teachers. The three experimental and two control groups received 7 hr of intervention spread out over two days. We assessed participants' pedagogical content knowledge before intervention, between the two days, after intervention, as well as at 6-week follow-up. The control groups exclusively received instruction on either pedagogical knowledge or pedagogical content knowledge; each of the experimental groups embodied a specific hypothesis about the formation of pedagogical content knowledge. Providing support for a mechanism of amalgamation, a combination of instruction on content knowledge and pedagogical knowledge produced small but statistically significant growth in pedagogical content knowledge. Similarly, instruction on content knowledge exclusively was sufficient to cause small but statistically significant growth in pedagogical content knowledge. Prior instruction on content knowledge did not facilitate learning from instruction on pedagogical content knowledge. Nevertheless, direct instruction on pedagogical content knowledge caused medium and statistically significant growth in pedagogical content knowledge. Educational Impact and Implications Statement We conducted a brief experimental study-interventions lasted 7 hr-with German preservice elementary school teachers to investigate the relevance of content knowledge and pedagogical knowledge for the formation of pedagogical content knowledge about teaching fractions. A combination of instruction on content knowledge and pedagogical knowledge, as well as exclusive instruction on content knowledge, produced small gains in pedagogical content knowledge. In contrast, despite preservice teachers' limited content knowledge, direct instruction on pedagogical content knowledge produced medium gains in pedagogical content knowledge. For elementary mathematics, the results suggest that teacher education programs that deliver early and explicit instruction on pedagogical content knowledge are more efficient in fostering pedagogical content knowledge than programs that provide instruction of content knowledge and pedagogical knowledge consecutively or that exclusively provide instruction on content knowledge.