The isochore structure of the nuclear genome of angiosperrasdescribed by Salinas et al. (1) was confirmed by using adifferent experimental approach, namely by showing that the levels of coding sequences from both dicots and Gramineae are linearly correlated with GC levels of the corresponding flankingsequences. The compositional distribution of homologous codingsequences from several orders of dicots and from Gramineae were also studied and shown to mimick the compositional distributions previously seen (l)for coding sequences in general, most coding sequences from Gcamineae being much higher than those of the dicots explored. These differences were even stronger for third codon positions and led to striking codon usages for many coding sequences especially in the case of Gramineae.