This study utilized an explanatory sequential mixed methods approach to explore randomly assigned treatment and control participants' frequency of inquiry instruction in secondary science classrooms. Eleven treatment participants received professional development (PD) that emphasized a structured approach to inquiry instruction, while 10 control participants received no PD. Two representative treatment participants were interviewed and observed to provide an in-depth understanding of inquiry instruction and factors affecting implementation. Paired t-tests were used to analyze quantitative data from observation forms, and a constant comparative approach was used to analyze qualitative data from surveys, interviews, purposeful observations and artifacts. Results indicated that treatment participants implemented inquiry significantly more frequently than control participants (p?<?0.01). Two treatment participants' instruction revealed that both used a similar structure of inquiry but employed different types of interactions and emphasized different scientific practices. These differences may be explained by the participants' understandings of and beliefs about inquiry and structuring inquiry. The present study has the potential to inform how methods of structuring inquiry instruction and teaching scientific practices are addressed in teacher preparation.