The status of three basic concepts of probability--points of a finite sample space, probability of a simple event in a finite sample space, and quantification of probabilities--possessed by children in grades four through seven was examined. A test for each of the three concepts was constructed by the author and administered to a total of 528 students who had not been taught probability previously. A multivariate analysis of covariance was performed on the results of the three tests, with grade equivalent scores on the Stanford Arithmetic Achievement Test used as covariates. The overall mean performances were significantly different among I.Q. groups, sex groups, and grades. The children demonstrated that they had acquired considerable knowledge about the three probability concepts without having received formal training. (Author/DT)