This study investigates the impact of a professional development programme for teachers that encourages more cognitively challenging, practical, and interactive science lessons. Using a Randomised Controlled Trial, its impact was measured by pupil learning outcomes. Pupils aged 9-10 at 42 primary (elementary) schools in Oxfordshire (England) were randomised to receive either the 'Thinking, Doing, Talking Science' (TDTS) programme, or to be in the business-as-usual control group. Two teachers per school attended five training days over the school year. These were designed to enhance their skills in providing conceptual challenge and improving pupils' higher order thinking by facilitating more discussion, more practical work and less (but more focused) writing in science lessons. The main outcome measure was an age-appropriate pencil-and-paper science test covering a range of topics and question types. Pupils also completed attitude surveys. Analysis of the scores for the 1264 pupils who took the pre- and post-tests shows this low-cost intervention had a statistically significant effect on attainment, with an effect size of +0.22. The impact was stronger among girls and slightly stronger among those with lower prior science attainment. Data from 1189 pupil surveys suggested that TDTS had also improved their attitudes to science.