"Preliminary demographic data are presented on the Datoga, a semi-nomadic pastoral population of northern Tanzania.  In comparison with other pastoral populations Datoga fertility is high.  There is a marked seasonal distribution of births that is only partly associated with rainfall patterns.  Survivorship chances up to the age of 15 are poor, and are independently affected by both length of the preceding interbirth interval and survivorship of the previous child.  The results are discussed in the context of ecological and social factors that affect fertility and offspring mortality rates in pastoral populations."