This report provides results of the first detailed examination of morphological abnormalities occurring in adults, pupae, and 4th-instar larvae of a sibling species complex within the family Culicidae. Aberrant and/or anomalous setae or structures are reported, along with their frequency of occurrence, for the 5 sibling species (Anopheles diluvialis, Anopheles inundatus, Anopheles maverlius, Anopheles quadrimaculatus, and Anopheles smaragdinus) of the Quadrimaculatus Complex of Anopheles. A total of 21,684 specimens were examined. In the pupal stage, the presence of seta 11-II was the most common anomaly, whereas in the 4th-instar larva, branching of seta 2-C occurred most frequently. Abnormalities in the male genitalia usually involved the claspette or tergum IX. Other abnormalities in females, male genitalia, pupae, and 4th-instar larvae occurred infrequently.