This paper presents a method of multi-parameter optimization of shell structures of constant thickness from linearly elastic homogeneous material. Our goal is to present how a relatively complex shape of shell can be optimized in terms of it stiffness only by changing its geometry and at the same time preserve the primary volume. Loading cases are not known exactly and so we use the criterion function which on the basis of arbitrary chosen eigenvalue frequencies, gives a stiffness of the shell structure. The optimization algorithm is based on stigmergy, which is becoming increasingly popular in combinatorial optimization and lately also in multi-parameter optimization. The method was verified on a real industrial problem on an electric motor casing.