This study illustrates the potential offered by observational methodology for analysing spontaneous behaviours and interactions by students with autism spectrum disorder (ASD). We present a case study in which we observed, analysed and interpreted how an adolescent boy with ASD interacted both verbally and non-verbally with his physical education teacher and with different gym- and pool-based tasks within a motor skills development programme. The data were annotated in the free software LINCE using an "ad hoc" observation instrument combining a field format and category systems. The quality of the data was confirmed by high levels of interobserver agreement (calculated quantitatively using Cohen's kappa statistic). Two diachronic techniques at the forefront of observational methodology, lag sequential analysis (GSEQ) and polar coordinate analysis (HOISAN), were used to investigate activating and inhibitory relationships corresponding to desirable and undesirable verbal and non-verbal interactions between the boy and the different zones in the motor skills circuits and the teacher's responses to these interactions. [Translation from English by Mercè Rius.]