Two or three isotypes of immunoglobulin (Ig) light (L) chain have been demonstrated in several teleost species thus far. A reverse transcription-polymerase chain reaction (RT-PCR) strategy was used with the aim of isolating cDNA clones encoding IgL chain from the common carp ( Cyprinus carpio. L.) different from the previously isolated isotype, designated L1A. cDNA clones representing two novel isotypes, designated L1B and L3, were obtained in this study. For CL segments, L1B sequences were similar to type L1/G in various teleost species, as well as L1A, while L3 sequences were closely related with type L3/F in catfish and zebrafish. For VL segments, however, L1A sequences demonstrated higher similarity to L3 rather than L1B. These results were also supported by the phylogenetic study. The similarity found between L1A and L3 VL sequences was based on the very high degree of nucleotide identity in FR1, and/or FR2 to FR3, strongly suggesting the involvement of interlocus recombination between them. Southern blot analyses suggested that the locus of L1B has a cluster-like organization, but that those of L3 and L1A are currently unknown due to cross-hybridization between those VL probes. Northern blot analyses showed that CL mRNA of each isotype was expressed in lymphoid tissues examined, particularly strongly in pronephros, and was predominantly composed of approximately 1-kb and 0.6-kb transcripts, possibly corresponding to fully spliced (LVJC) and truncated (JC/J-C) forms, respectively.