Crepis japonica (L.) Benth., Porophyllum ruderale (Jacq.) Cass. and Tridax procumbens L. are weedy species that grow in cultivated fields, roadsides, abandoned fields and open, disturbed spaces in Maringa, Parana state, Brazil. The ontogeny of the fruits and seeds of the 3 Asteraceae species was carried out. The flowers and developing fruits were prepared according to resin inclusion techniques for histochemical tests and scanning electron microscopy. During maturation of the pericarp, processes such as trichome differentiation, tissue sclerification, phytomelanin deposition and breakdown of tissues can be observed. The fruit of C. japonica is entirely lacking in phytomelanin. The seed is only exotestal in C. japonica. Comparative analysis of the pappus and cypsela characters demonstrated that they are efficient in separating species and tribes. The seed coat and embryo may also be useful in the characterization of tribes.