The aim of this study was to determine the egg laying period of Gasterophilus nasalis and to examine whether there is a relationship with the climatic conditions of the area. Every two weeks, from November 2002 to March 2003, the number of eggs laid in the submaxilar region of 10 Hackney horses of a farm near Yumbel, in the 8th Region of Chile (37°, 03', S.; 72°, 33', O.), was determined. The horses were kept in a pasture without antiparasitic treatment during the study. After each egg count, a sample of eggs was removed for laboratory analysis while the remaining eggs were stained with an aqueous solution of methylene blue in order to distinguish them from those eggs laid during the following two weeks. Climatic data were obtained from a meteorological station close by. G. nasalis was found to begin laying its eggs at the end of November 2002. The quantity of eggs laid in the intermandibular region of the 10 horses, peaked in mid December 2002 (853 eggs) and again in mid January 2003 (945 eggs). Afterwards, the quantity of eggs laid dropped to 300 every two weeks until the beginning of April and ended completely in May 2003. The egg laying period of G. nasalis coincided with mean temperatures over 15°C. Rainfall negatively influenced the egg laying of the parasite. In March and April 2003, a consistent increase in the numbers of eggs laid by G. intestinalis was detected surprisingly in the chest, shoulder joint, loin region and on the front and hind legs. More than 2000 eggs were counted in the 10 animals, confirming the presence of G. intestinalis in Chile. The moment when G. intestinalis stopped laying eggs and the existence of a relationship with climatic conditions were not able to the confirmed. It was concluded that in Chile, G. nasalis lays eggs from the end of November until the beginning of May, coinciding with mean temperatures over 15°C., and that G. intestinalis begins the oviposition at the beginning of March.