This embedded mixed method study used a randomised controlled? trial design to examine two cohorts of upper (grades 4-6) elementary teachers' classroom implementation of nature of science (NOS) instruction following their participation in a statewide professional development (PD). The treatment group (n = 145) was compared to a control group (n = 90), which received no PD. The PD included a summer institute that situated explicit NOS instruction within the context of problem-based learning (PBL). Data sources included videotaped classroom observations across four time points and teacher-generated descriptions of lessons preceding and following the observed lesson including learning objectives. Data were analysed with descriptive and inferential statistics and analysed qualitatively for trends in participants' explicit NOS instruction. Results indicated the majority of treatment teachers (n = 97; 66.9%) accurately taught explicit NOS during the academic year following the summer institute compared to control teachers (n = 2; 2.2%). Participants most often taught about the empirical (69%) and social (63%) aspects of science. Participants' strategies for integrating NOS varied in temporal placement within the lesson, connectedness to lesson activities, and extent of student-centeredness. These results demonstrate that situated PD that contextualises explicit NOS instruction within PBL facilitated teachers' explicit NOS instruction compared to teachers who did not receive such instruction.