Background: Games represent a very important part of the physical education curriculum and the process by which they are learnt is very complex. Constructive teaching theories highlight the existence of knowledge prior to instruction that the pupil actively transforms through verbalisation and interaction with classmates. The results of research studies conducted from the learning perspective reveal wide variations in the knowledge possessed by students, their understanding of the game and motor skills, and the broad range of tactical solutions in a single game situation in accordance with their knowledge of the specific domain. In general terms, most participants in research studies on prior knowledge and the specific domain knowledge have basic knowledge, the level of which is closer to beginners, as well as difficulty in making decisions based on a prior assessment of the game context. Purpose: The aim of this research study is to gauge the declarative and procedural knowledge of the students as well as their performance in a modified invasion game. Participants and setting: A total of 27 fourth-year primary-school students (19 boys and eight girls aged 9-10 years) from an intact group at one public school took part in the study. Research design: A descriptive design was used to measure tactical knowledge and performance in an invasion game. Data collection: Students performed an invasion games knowledge test and were then divided into teams with a similar level of invasion game knowledge for 3 on 3 play in a modified invasion game which was videotaped. Six of the students (three boys and three girls) were then interviewed in order to gauge their invasion games knowledge. As soccer is the most widely played invasion game in Spain, it was used in video sequences as the invasion games content to analyse student's knowledge. Data analysis: Repeated measures ANOVA was conducted to analyse differences in variables. Results obtained from the interview were descriptively analysed. Findings: At initial stages of games learning, students had difficulties in using their tactical knowledge in game play. Game performance was higher when students advance on goal than when students kept possession of the ball. No correlation was observed between the knowledge possessed by students and the correct decisions made in relation to the technical/tactical aspects of the game. Conclusions and implications: The results reveal that fourth-year primary-school students attending PE classes have a prior knowledge of invasion games. Determining tactical knowledge and performance by means of a modified invasion game is important in implementing quality games-teaching programmes for schoolchildren. Linking tactical knowledge and performance to the game contexts that students have to resolve is thus an efficient way of investigating and identifying knowledge of the specific domain and students' previous sporting experiences. (Contains 3 tables.)