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During the last decades, massive blooms of the toxic benthic dinoflagellate Ostreopsis cf. ovata have occurred in different coastal areas of the Mediterranean Sea typically during the late summer period. O. cf. ovata blooms have been related to human intoxication and to mass mortalities of benthic invertebrates due to the production of palytoxin-like compounds, including isobaric palytoxin, ovatoxins and mascarenotoxins. In the last decades, several microalgae have been shown to release a wide range of secondary metabolites that mediate interactions between individuals of different species. A variety of diatoms genera produce and release different types of fatty acid derived long-chained polyunsaturated aldehydes (PUAs). These compounds have been related to multiple functions such as adverse effect on the reproduction of various invertebrates, intra- or interspecific signals, and have effects on a variety of marine organims as well, including co-occurring microalgae. Moreover, several studies have highlighted changes on growth, cell membrane permeability, flow cytometric properties and cell morphology due the exposition of microalgae to PUAs. The relationships between O. cf. ovata and co-occurring microalgae has been previously investigated (Pichierri et al., 2014), showing that some benthic diatoms exherted a negative effect on the growth rate of O. cf. ovata. The aim of this study was to investigate the effects of three toxic algal-derived PUAs, namely the 2E,4E-decadienal, 2E,4E-octadienal and the 2E,4E heptadienal, on the growth and cell morphology of O. cf. ovata. O. cf. ovata was cultured under controlled laboratory conditions at different PUAs concentrations (from 0.1 to 36 µmol l−1) and using two different exposition times (24 and 72 h). Our results showed a clear decrease of O. cf. ovata growth when cultures were exposed to the various PUAs compared with the control. After 72 h of exposition at the maximum concentration tested, the growth decrease respect to the control was 92.4%, 81.5% and 48.6% for 2E,4E-decadienal, 2E,4E-octadienal and 2E,4E heptadienal, respectively. Although a concentration-dependent decrease of the growth rate was observed for all three aldehyde compounds, the effect was stronger with longer-chained molecules than with shorter-chained ones. Moreover, at the highest concentrations of 2E,4E-decadienal (from 6 to 36 µmol l−1), aberrant forms of O. cf. ovata cells were observed, showing a marked decrease of cell dimensions, a contraction of cytoplasm and the formation of abnormal vesicle-like structures. These data pointed out for the first time the negative effect of PUAs, which are compounds produced by some diatoms species, on the growth and morphology of O. cf ovata cells and confirm the possible allelopathic role of these molecules in the relationships among phytoplankton species. Further studies are needed to better clarify the interactions between diatoms and O. cf. ovata, especially on bloom-forming dynamics.