Freshwater　ecosystems　in　high-mountain　regions　are　subject　to　emerging　threats　such　as　global　warming　and　expanding　human　activities.　Stream　diatoms　and　macroinvertebrates　form　an　essential　component　of　freshwater　ecosystems　in　high-mountain　regions.　Although　these　organisms　are　sensitive　to　environmental　changes,　knowledge　regarding　their　elevational　diversity　patterns　remains　limited.　Opposite　aspects　(e.g.,　north　vs　south;　west　vs　east)　usually　receive　different　amounts　of　solar　radiation　and　precipitation,　leading　to　distinct　in-stream　characteristics　such　as　discharge,　flow　regime,　and　water　temperature.　Despite　the　suggested　strong　influence　of　aspect　on　biodiversity　patterns　in　mountains,　its　effect　on　stream　diatoms　and　macroinvertebrates　has　been　largely　overlooked.　The　aims　of　our　study　were　to　1)　investigate　whether　macroinvertebrate　and　diatom　taxon　richness　follows　the　same　pattern　along　an　elevational　gradient;　2)　test　the　effect　of　aspect　on　the　elevational　diversity　(i.e.,　taxon　richness　and　assemblage　dissimilarity)　patterns　of　macroinvertebrate　and　diatom　assemblages;　and　3)　examine　the　relative　importance　of　elevation,　aspect,　and　the　local　environment　(e.g.,　in-stream　physicochemical　variables)　in　shaping　macroinvertebrate　and　diatom　assemblages.　We　investigated　macroinvertebrate　and　diatom　assemblages　in　six　nearly　parallel　streams　(three　streams　on　the　east　aspect　and　three　on　the　west)　in　the　Hengduan　Mountains　region.　We　found　that　the　taxon　richness　of　both　macroinvertebrates　and　diatoms　showed　a　monotonic　increase　with　elevation　(1623–2905　m　a.s.l.)　when　aspect　was　not　accounted　for.　When　aspect　was　taken　into　consideration,　macroinvertebrate　taxon　richness　still　showed　a　monotonically　increasing　elevational　pattern　on　both　the　east　and　west　aspects,　but　with　significantly　different　model　slopes,　while　a　monotonical　pattern　for　diatoms　only　remained　on　the　west　aspect.　In　addition,　taxon　richness　of　macroinvertebrates　may　also　follow　a　potential　unimodal　pattern.　The　distance-decay　relationships　followed　the　same　patterns　as　taxon　richness,　suggesting　that　the　influence　of　aspect　on　diatom　and　macroinvertebrate　assemblages　was　not　negligible.　Our　results　suggested　that　the　diversity　of　both　diatoms　and　macroinvertebrates　was　jointly　determined　by　elevation,　aspect　and　local　environment.　Compared　to　macroinvertebrates,　the　diatom　assemblages　were　more　strongly　influenced　by　the　local　environment.　To　gain　a　better　understanding　of　the　underlying　mechanisms　driving　elevational　patterns　of　stream　biodiversity　in　high-mountain　regions,　comparative　studies　that　involve　multiple　organisms,　streams,　and　mountains　across　a　large　elevational　range　are　needed.　©　2019　Elsevier　Ltd