The capsids of single-stranded RNA bacteriophages show remarkable structural similarity. In an attempt to test whether the coat protein (CP) from one bacteriophage could substitute for the CP of another and form mixed particles, we reassembled capsids in vitro from a mixture of different RNA phage CP dimers together with E. coli ribosomal RNA. Surprisingly, mixing CPs from phages belonging to groups I and II led to appearance of rod-like particles along with icosahedral spherical capsids, both containing a mixture of the two CPs. Rods and mixed spherical capsids containing host RNA were also obtained in vivo in bacteria expressing simultaneously fr and GA CPs. In a co-infection of the two phages, however, only authentic fr and GA virions were formed. Coat protein mutants in the FG loop were unable to assemble into rods, suggesting that these loops are involved in the formation of the aberrant particles.