The effects of a polyamine, spermine, on the differentiation of Naegleria gruberi amebas into flagellates were tested. Addition of spermine at early stages of differentiation (until 40 min after the initiation of differentiation) completely inhibited the differentiation. To understand the inhibition mechanism, we examined the effect of spermine treatment on the transcription and translation of differentiation-specific genes during differentiation. Addition of spermine at early stages did not inhibit the accumulation of two differentiation-specific mRNAs, ${alpha}$-tubulin and Class I mRNA, significantly, but rather prevented the rapid degradation of the mRNAs in later overall protein synthesis partially and gradually. However, translation of the ${alpha}$-tubulin mRNA was completely inhibited. These data suggest that the inhibition of differentiation of N. gruberi by spermine treatment did not result from the inhibition of transcription of differentiation-specific genes but from the specific inhibition of translation of the mRNAs during the differentiation.