Goats are held in large numbers in Oman and are widely appreciated for their meat. Infectious diseases are important risk factors that limit their production and increase the cost of management. The study of the caprine immune system is therefore crucial for an understanding of the host - parasite interaction in this species. Generally, It is assumed that heat stress, leads to suppression of the immune system and therefore increases the susceptibility of these animals to disease. However, based on a number of observations at the field level, it appears that morbidity and mortality figures in goats increase for a significant number of diseases during the winter months of the year versus the summer months. This study is, therefore, an attempt to investigate the effect of seasonal variations on different immune system parameters, including the bactericidal activity of caprine polymorphonuclear granulocytes (PMNs) against the target bacteria Staphylococcus aureus (Staph. aureus) and the production of serum immunoglobulins. In addition, serum cortisol, total protein and hematological values were measured. Six healthy male goats approximately one year of age were maintained together in a closed pen at the Agriculture Experimentation Station at Sultan Qaboos University for a period of one year. The goats were fed Rhodesgrass hay ad libitum and a commercially prepared ruminant concentrate. Blood samples were collected monthly for hematological, serological and cortisol analysis, as well as to study the phagocytic activity of PMNs. An automated weather station was used for meteorological monitoring of solar radiation, air temperature, relative humidity, rainfall and wind speed as well as photoperiod. The average ambient temperature during the winter (October – March) was 24 0.3°C versus 34 + 0.3°C in the summer (April – September) (P<0.05). The photoperiod was significantly shorter in the winter versus the summer, 9.6 + 0.5 and 11.3 + 0.8, respectively. During summer, there was an increase in the level of serum IgG (P<0.05), and an increase in the phagocytic activity of PMNS against Staph. aureus (P<0.05). Also during the summer season there were significantly lower levels (P<0.05) of serum cortisol (26.5 + 2.2) than that recorded during the winter season (34.9 5.1). During the summer the goats had higher WBC (P<0.05), lymphocyte (P<0.05) and eosinophil counts (P=0.05) than during the winter season. There were no statistical differences in neutrophil (P=0.32), monocyte (P=0.47) and basophil counts (P=0.7) between the two seasons. The seasonal differences in phagocytic activity and levels of IgG could have been partially related to varying cortisol levels. However, it could also have been related to differing photoperiods during the two seasons. The increase in the eason incidence of various infectious diseases during the winter season in goats in Oman may be at least partially due to suppressed phagocytic and/or opsonisation activity.