The dynamics of immunoglobulin G (IgG) subclasses (IgGa, IgGb and IgGc) were evaluated in foals infested by Gasterophilus spp., in an oceanic climate area (Northwest (NW) Spain). During a 1-yr period, blood samples were collected from sixteen 3-month-old foals, born in February, and their sera analyzed by using an enzyme-linked immunosorbent assay (ELISA) and excretory/secretory antigens from G. intestinalis second-stage larvae (GphiL2ES). With the purpose of assessing the possible effect of reinfestation on the humoral immune response, one group of eight foals was treated with moxidectin at the end of August. The antibody response increased during the summer, then decreased slowly till October and finally the highest values were observed in December; a marked reduction was observed in January and then the values decreased progressively till the end of the study (April). The kinetics of IgGa response registered higher values throughout the study, and the IgGb and IgGc recorded weak responses. Based on the high values that were recorded for the IgGa throughout the study, this response was analyzed to define the life-cycle of G. intestinalis. The increment in IgGa levels observed in June-July was related to the primary infestation of the foals; the higher values recorded between August and December, with the development of the active phase of the endogenous cycle; and the reduction from December to April, with a lessening in the numbers of L3s due to these instars exiting along with the feces. A similar response was recorded during reinfestation in the foals.