I have recently isolated a bacteriophage for E. coli from soil. The phage has been purified through 6 rounds of serial dilution and has given consistent plaques after the second purification. That is why my professor and I consider it as isolated. The only issue is that it always displays two distinct plaques: a large, clear one of about 0.5 cm and a smaller clear one of about 0.2 cm. There are no sizes in between. It is either or. When I tried to take bacteriophages from the small and from the large plaques, I resulted in the exact same two plaque sizes. Taking from these as well still gave one large and one small. It has been my understanding a phage gives only one unique plaque size which is dependent on absorption rate into the cell, not two distinct plaque sizes. Does anyone have any ideas why this is happening?