A Monte Carlo simulation is used to study the formation of finite-size nanostructures in a dilute salt-free solution of stiff polyanions in the presence of short cationic diblock chains consisting of neutral tails and charged heads. At strong electrostatic interaction, the system is found to undergo the self-organization resulting in the formation of planar aggregates that look like a 'ladder' of polyanions sandwiched between cationic chains. We investigate the stability of different morphologies and find that these aggregates are thermodynamically stable. (c) 2005 Elsevier B.V. All rights reserved.