Summary Two major types of F rankia strains are usually recognized, based on the ability to sporulate in‐planta : spore‐positive ( Sp +) and spore‐negative ( Sp −). We carried out a study of Sp + and Sp − Frankia strains based on nodules collected on A lnus glutinosa , A lnus incana and A lnus viridis . The nodules were phenotyped using improved histology methods, and endophytic F rankia strain genotype was determined using a...