As Mike wrote above, the improved Meyer lemon is a virus free form of the original Meyer lemon. In the 1940's the original Meyer lemons were found to be a carrier of the tristeza virus. As a carrier of the fatal virus, it passed the virus to other citrus varieties, but never shown any of the symptoms itself. The propagation and sale of the original Meyer lemon was banned 20 or 30 years ago. Luckily in the early 50's a virus free tree was found by none other than Don Dillon, the founder of the Four Winds Growers dwarf citrus company, a very popular California dwarf citrus tree propagator and supplier. I have purchased many excellent quality citrus tree from them. After Don's tree was certified by the University of California to be indeed free of tristeza, it was release for propagation and sale to the general public. The Improved Meyer lemon has mostly been grown only as a back yard tree, and never as a commercial tree, as its fruit does not holdup well under long distance shipping. We owe word of thanks to Don Dillon for all the Improved Meyer lemons we are able to enjoy today. - Millet