In the last 30 years, there has been an ongoing discussion about the effectiveness of Professional Development (PD) programs, which aims to promote reform-based Science Education (SE). Among the many, different trends of reform-based science teaching, inquiry-based approaches hold a dominant role. This study shows how teachers' practices were affected by a PD program that aimed to familiarize them with reform-based teaching through gradual instructional design, with the main focus on inquiry. The PD program had a duration of 12 months and involved four science teachers (two primary and two secondary) who were trained in both in and out of school teaching settings. The changes in teachers' practices were recorded through an observation protocol containing predefined categories in eight domains, one of which--that of inquiry--is discussed in this paper. A semi-quantitative method was used for data analysis. Results indicate that all the teachers had an overall improvement in the domains of guided inquiry practices and student-centered teaching approaches. However, there did not appear to be any substantial progress in open inquiry practices. Restrictions of the present study are presented, and suggestions for improving future PD programs promoting sustainable inquiry implementation are also discussed.