Qualitative and quantitative aspects of fetal behaviour were studied in nine healthy nulliparae during the second half of pregnancy. Continuous real-time ultrasound observations of 60 min duration were carried-out at four-weekly intervals from 20 to 36 weeks of gestation, using two dynamically focussed linear array transducers positioned lengthwise allowing viewing of sagittal sections through the entire fetus. Various movement patterns were classified according to de Vries et al. (de Vries, J.I.P., Visser, G.H.A. and Prechtl, H.F.R (1982): The emergence of fetal behaviour I. Qualitative aspects. Early Hum. Dev., 7, 301-22). The reproducibility as expressed by the mean standard deviation in the analysis of the percentage incidence of the different movement patterns was good (1.6-2.2%). Both the incidence and percentage of observation time in which general movements were present decreased with advancing gestational age. Breathing movements frequently occurred as an isolated phenomenon; occasionally they were associated with jaw movements, swallowing, hiccups or general movements. The incidence of breathing movements as well as the percentage of observation time in which breathing movements were present, demonstrated an increase up to 32 weeks with a drop at 36 weeks of gestation. Hiccups occurred episodically, following each other in regular succession. Both the incidence and the percentage of observation time in which hiccups were present fluctuated widely. Stretches and startles displayed a decrease in incidence with advancing gestational age, whilst eye movements showed an increase. Rhythmical jaw movements were often followed by swallowing. Occasionally, yawns or tongue movements were seen. They may be combined with breathing and general movements. The incidence of jaw movements increased up to 28 weeks with a decline after. Both brief and longer periods of hand-face contacts were documented with wide variation in incidence. Head rotations frequently occurred as a single event; the incidence showed a decrease up to 28 weeks followed by an increase at 36 weeks of gestation. The incidence of head retroflexion varied widely with gestational age.