Few, if any, studies have examined the impact of nature of science (NOS) instruction on science teachers' practices 2 or more years after completing a science teacher education program. Extant studies on preservice and first-year teachers' NOS teaching practices have had disappointing results, with few teachers valuing NOS as a cognitive objective or teaching it in ways consistent with literature regarding effective NOS instruction. In addition, little is known about teachers' specific NOS practices due to a lack of observation protocols to assess teachers' NOS instruction. This study examined teachers' NOS instructional practices 2-5 years after completing an intensive secondary science education program that included a NOS course and attention to NOS instruction throughout all other science education coursework. Twelve of the 13 study participants explicitly taught NOS, and 9 of the 13 did so at moderate to high levels. This paper also presents a NOS Classroom Observation Protocol (NOS-COP) designed to make evident many facets of teachers' NOS implementation practices that have not always been clear in prior research. This study raises important issues about achieving the goal of NOS instruction. Accurate and effective NOS instruction appears achievable, but may require far more effort than found in typical science teacher education programs. (Contains 6 tables and 3 figures.)