Two studies were conducted to investigate, firstly, children's focusing on the aspect of numerosity in utilizing enumeration in action, and, secondly, whether children's Spontaneous Focusing on Numerosity (SFON) is related to their counting development. The longitudinal data of 39 children from the age of 3.5 to 6 years showed individual differences in SFON, as well as stability in children's SFON across tasks during the follow-up. Path analyses indicated a reciprocal relationship between SFON and counting development. The results were confirmed by a cross-sectional study of 183 6.5-year-old children when the effects of non-verbal IQ, verbal comprehension and lacking enumeration and procedural skills were controlled.