Circadian　clocks　control　the　rhythmicity　of　many　behaviors　and　physiological　features　of　insects.　To　study　the　circadian　clock　of　the　moth　Plutella　xylostella,　we　employed　CRISPR/Cas9-mediated　genome　editing　to　investigate　the　effect　of　loss　of　the　clock　gene　period　on　the　circadian　rhythms.　P.　xylostella　harbors　a　single　copy　of　period.　Phylogenetic　analysis　showed　that　P.　xylostella　PERIOD　is　more　homologous　to　mouse　PERIOD　than　the　PERIOD　proteins　from　bees,　flies,　mosquitos,　and　many　other　Lepidoptera,　such　as　Danaus　plexippus　and　Bombyx　mori.　The　circadian　rhythms　in　adult　locomotor　activity　were　altered　in　the　period　knockout　strain　of　P.　xylostella　under　light–dark　(LD)　and　continuous　dark　(DD)　conditions.　Under　the　LD　cycle,　the　wild-type　moths　displayed　nocturnal　activity　with　activity　peaking　very　early　after　lights　off　and　quickly　declining　after　lights　on.　In　contrast,　the　period　knockout　strain　had　no　peak　in　activity　when　the　lights　were　turned　off　and　exhibited　steady　activity　throughout　the　hours　of　darkness.　Interestingly,　under　DD　conditions,　our　results　showed　that　the　locomotor　rhythm　can　be　maintained　without　period　gene,　but　at　a　lower　rhythmicity　ratio　than　wild-type.　In　addition,　knockout　of　period　in　P.　xylostella　changed　circadian　rhythms　patterns　related　to　pupal　eclosion,　mating,　egg-laying,　and　egg　hatching.　Mechanistically,　loss　of　PERIOD　disrupted　the　molecular　rhythm　of　period　and　changed　the　clock　transcription　rhythm　in　the　heads　of　the　moths　under　LD　and　DD　conditions.　Together,　our　study　indicates　that　the　PERIOD　is　required　for　normal　expression　of　many　behavioral　rhythms　in　P.　xylostella.　©　2022　Institute　of　Zoology,　Chinese　Academy　of　Sciences.