The European corn borer (ECB),Ostrinia nubilalis(Hübner), is a major pest of corn in many countries. To quantify borer losses, a model describing development of the five instars of ECB for a single generation of development, tested on data collected at INRA-Versailles, France, is proposed and discussed. It consists of two parts: a degree-day model that takes into account the influence of temperature; and normal distributions that represent the variability of population development time among instars. A method of estimating the parameters of this model for field data by maximum likelihood is described. This statistical approach showed how the experimental design should be improved. For the six climatic sequences recorded, this simple model is an appropriate representation of development of the four instars sampled with enough accuracy. A validation of the model supports this conclusion. The chosen model is based on simple biological hypotheses and can be fitted under field conditions. The advantages of these two characteristics lies in the possibility of comparing laboratory and field estimations, to analyze their differences, and to determine under which environmental conditions the model is correct (i.e., validity area).