Herein,　the　particle　size　distributions　(PSDs)　and　shape　analysis　of　in　vivo　bioproduced　particles　from　aqueous　Au3+　and　Eu3+　solutions　by　the　cyanobacterium　Anabaena　sp.　are　examined　in　detail　at　the　nanoscale.　Generally,　biosynthesis　is　affected　by　numerous　parameters.　Therefore,　it　is　challenging　to　find　the　key　set　points　for　generating　tailored　nanoparticles　(NPs).　PSDs　and　shape　analysis　of　the　Au　and　Eu-NPs　were　performed　with　ImageJ　using　high-resolution　transmission　electron　microscopy　(HR-TEM)　images.　As　the　HR-TEM　image　analysis　reflects　only　a　fraction　of　the　detected　NPs　within　the　cells,　additional　PSDs　of　the　complete　cell　were　performed　to　determine　the　NP　count　and　to　evaluate　the　different　accuracies.　Furthermore,　local　PSDs　were　carried　out　at　five　randomly　selected　locations　within　a　single　cell　to　identify　local　hotspots　or　agglomerations.　The　PSDs　show　that　particle　size　depends　mainly　on　contact　time,　while　the　particle　shape　is　hardly　affected.　The　particles　formed　are　distributed　quite　evenly　within　the　cells.　HR-PSDs　for　Au-NPs　show　an　average　equivalent　circular　diameter　(ECD)　of　8.4　nm　(24　h)　and　7.2　nm　(51　h).　In　contrast,　Eu-NPs　preferably　exhibit　an　average　ECD　of　10.6　nm　(10　h)　and　12.3　nm　(244　h).　Au-NPs　are　classified　predominantly　as　“very　round”　with　an　average　reciprocal　aspect　ratio　(RAR)　of　~0.9　and　a　Feret　major　axis　ratio　(FMR)　of　~1.17.　Eu-NPs　mainly　belong　to　the　“rounded”　class　with　a　smaller　RAR　of　~0.6　and　a　FMR　of　~1.3.　These　results　show　that　an　increase　in　contact　time　is　not　accompanied　by　an　average　particle　growth　for　Au-NPs,　but　by　a　doubling　of　the　particle　number.　Anabaena　sp.　is　capable　of　biosorbing　and　bioreducing　dissolved　Au3+　and　Eu3+　ions　from　aqueous　solutions,　generating　nano-sized　Au　and　Eu　particles,　respectively.　Therefore,　it　is　a　low-cost,　non-toxic　and　effective　candidate　for　a　rapid　recovery　of　these　sought-after　metals　via　the　bioproduction　of　NPs　with　defined　sizes　and　shapes,　providing　a　high　potential　for　scale-up.　©　2022　by　the　authors.