Several rotifers including
Philodina
spp. are well known to make commensal and parasitic associations with different animals. The present investigation was carried out to decipher the relationship of
Philodina roseola
with a piscine ectoparasite
Argulus bengalensis
in its embryonic stage. Mechanical removal of the symbiont
P. roseola
from the argulid egg strips resulted in the complete (100%) failure in hatching. Several
P. roseola
individuals were found to feed on the solidified jelly coat of the eggs enabling the larvae to emerge under both laboratory and field conditions. Under the laboratory condition, the experimental removal of
P. roseola
did not affect the embryonic development, but it rendered the jelly coat intact; therefore, the larvae were unable to make hatching furrow and subsequently died. The results of our experiments thus prove this service–resource relationship to be a mutualism. Although the association is facultative for
P. roseola
, it is obligatory for
Argulus
spp. An act of intervention in this relationship thus offers a promising control of argulosis.