Developing both conceptual and procedural knowledge is important for students' mathematics competence. This study examined whether U.S. Grade 9 general education mathematics teachers' self-reported use of concept-focused instruction (CFI) and procedure-focused instruction (PFI) were associated differently with ninth graders' algebra achievement after 2.5 years, depending on students' mathematics difficulty (MD) status. Data for this study were drawn from the High School Longitudinal Study for the years 2009-2010 and 2011-2012 (N = 19,104). Multiple regression analyses indicated that students with MD who participated in Grade 9 mathematics classrooms where teachers self-reported the use of less CFI and more PFI were more positively associated with having higher algebra achievement after 2.5 years. Conversely, students without MD in classrooms where mathematics teachers self-reported the use of more CFI and less PFI were positively associated with having higher algebra achievement after 2.5 years. However, this study's findings do not suggest that teachers should disregard CFI and provide only PFI when teaching students with MD. Because the data set did not include any variable to discover whether teachers provided sufficient support (i.e., evidence-based practices) for students with MD, who have more constraints in their cognitive skills compared to students without MD, to benefit from CFI, the findings of this study should be interpreted cautiously. Directions for future research and practical implications are discussed.