The mature larvae of Phaedon (Orthosthica) semimarginatus affinis Guérin is described and illustrated. This larva shares several morphological features with other members of the genus. It is similar to P. concinus Stephens and P. conchleariae (F.) in that tubercles Es and Ss are fused on meso and metathorax and the absence of Dae in segments 1–6. Frontal setae (f 2) are absent on larvae of P. (O.) s. affinis, tubercles Dai and Dae are separated on meso and metathorax, and only tubercle Dai is present on abdominal segments 1–3. Natural history is also discussed.