The development of social behavior in juvenile guanacos (chulengos) was studied by focal animal observation in Torres del Paine National Park for one annual cycle. This is the first study of its kind on a free ranging, wild population of guanacos. Birthing, mating, expulsion, formation of winter groups, and migration were key events influencing chulengo social development. The highest rate of alarm behavior occurred in January when chulengos were one month old. The percentage of chulengos nursing/hr decreased from a high of 56% in January to a low of 0% in July and 0% in September. There was no difference between the sexes in duration of nursing during summer or fall, but in winter chulengo males nursed more. Amount of time spent in different activities varied between summer and winter: feeding increased from 30% to 93% while resting decreased from 50% to 25%, walking from 12% to <5%, and playing decreased from 5% to 0%. Chulengos showed submissive behavior towards adult males throughout the year, with significant peaks in May when in Mixed Groups and in November during expulsion from the Family Group.