ABSTRACT. The chaetotaxy and structures of the head, thorax and abdominal segments VIII and X of the first 3 instars of Aedes aegypti (Linnaeus) and A. seatoi Huang, are tabulated and illustrated. Characters are tabulated that will permit identification of the 4 larval stages within each species and differentiate the first 3 larval instars of aegypti and seatoi. A serial acquisition of certain thoracic setae is noted that will consistently separate second and third stage larvae of these species and at least two species in other genera. These setae will probably be very useful in deriving setal homologies, and may also represent a general character for differentiating second and third stage larvae.