To gain insight into the functions of the nuclear-encoded factors involved in chloroplast development, we characterized the high chlorophyll fluorescence and pale green mutant 108-1 (designated as hfp108-1) of Arabidopsis thaliana. Map-based cloning revealed that the mutant contains a tandem repeat of part of the sequence (including 116 nucleotides from 631 to 746 bp downstream of the ATG) of At3g63190, which encodes a chloroplast ribosome recycling factor homologue and was named AtcpRRF. The chloroplasts of hfp108-1 plants contain few internal thylakoid membranes and are severely defective in the accumulation of chloroplast-encoded proteins. In vivo labeling experiments showed a drastic decrease in the synthesis of the chloroplast-encoded proteins, which may be attributed primarily to reduced translation of the corresponding mRNA molecules. The level of the HFP108 transcript was greatly reduced in hfp108-1, so hfp108-1 showed a weak phenotype, and null alleles of HFP108 (hfp108-2) were embryonic lethal. Observations with cleared seeds in the same silique showed that homozygous hfp108-2 seeds were blocked at the heart stage and did not develop further. Thus, these results suggest that AtcpRRF is essential for embryogenesis and chloroplast biogenesis. ined