Circular RNAs (circRNAs) are abundant and evolutionarily conserved RNAs of 
largely unknown function. Here, we show that a subset of circRNAs is translated 
in vivo. By performing ribosome footprinting from fly heads, we demonstrate that 
a group of circRNAs is associated with translating ribosomes. Many of these 
ribo-circRNAs use the start codon of the hosting mRNA, are bound by 
membrane-associated ribosomes, and have evolutionarily conserved termination 
codons. In addition, we found that a circRNA generated from the muscleblind 
locus encodes a protein, which we detected in fly head extracts by mass 
spectrometry. Next, by performing in vivo and in vitro translation assays, we 
show that UTRs of ribo-circRNAs (cUTRs) allow cap-independent translation. 
Moreover, we found that starvation and FOXO likely regulate the translation of a 
circMbl isoform. Altogether, our study provides strong evidence for translation 
of circRNAs, revealing the existence of an unexplored layer of gene activity.