Abstract Baier, B., Lamml, M. and Kramer, B. 2006. Ontogeny of the electric organ discharge in two parapatric species of the dwarf stonebasher, Pollimyrus castelnaui and P. marianne (Mormyridae, Teleostei). —Acta Zoologica (Stockholm) 87: 209 –214 This is the first description of the ontogenetic development of the electric organ discharge (EOD) in two parapatric mormyrid sibling species, Pollimyrus castelnaui (Boulenger, 1911) and P. marianne Kramer et al. 2003, from the first appearance during the early larval stages at about 0.7 cm total length to the fully developed adult discharge at about 3.0 cm total length. At around day 20 the biphasic larval discharge (which is similar to that of P. adspersus larvae) is followed by the emerging triphasic adult discharge with a delay of about 0.7 ms. Both discharge types coexist for about 3 weeks. As the development advances, the amplitude of the adult discharge increases rapidly while that of the larval discharge diminishes. Whereas the adult EOD waveform of P. marianne remains triphasic later in life, in P. castelnaui an ‘initial flourish’, in the form of additional small phases, is added and develops at an age of about 40 days, when larval and adult EOD are present. Possible evolutionary scenarios are discussed.