Descripción: We examined the structure and characteristics of alarm calls uttered by nesting Southern House Wrens (Troglodytes musculus) during the 2007-2008 breeding seasons. We took multiple structural measures of 221 calls uttered by breeding individuals and used multivariate analyses to classify them. We recognized two distinctive alarm calls, named Type I and Type II. Type I calls were longer in duration and had higher frequencies than Type II calls; however, birds used Type II calls at a higher rate. Given the structural differences found between both call types; we propose that each call type has a different function, which remains to be tested. © The Neotropical Ornithological Society.