Inclusion Criteria:  A diagnosis of PKU on new-born screening requiring a low protein diet and Phe-free L-amino acid protein substitute. Aged between 6 months and 3 years (inclusive at screening). Already taking the majority of their protein substitute in a spoonable form OR is at the stage in their PKU management when a second stage spoonable protein substitute is recommended to commence. Well-controlled PKU, evidenced by the latest three routine blood spots being within the acceptable range, in the investigator's opinion. Able to comply with the study protocol and take the study product, according to the opinion of the investigator. Willingly given, written, informed consent from parents/guardian.  Exclusion Criteria:  Diagnosis of persistent hyperphenylalaninaemia, or mild PKU not requiring dietary intervention with a low protein diet and Phe-free L-amino acid supplements. Diagnosis of a concurrent condition which may adversely affect developmental progression and feeding ability. Known milk or fish allergy/intolerance. Patients who are currently participating in, plan to participate in or have participated in an interventional investigational drug, food or medical device trial within 30 days prior to the screening visit. Existing significant GI issues which may affect compliance with the study protocol, according to the opinion of the investigator. Any medical conditions precluding the study intervention, which in the opinion of the investigator may impact on metabolic control during the study period. Use of additional macro/micronutrient supplements during the study period, unless clinically indicated and prescribed by the investigator (must be recorded in patient case record file). Where applicable, patients NOT covered by Health Insurance System and/or not in compliance with the recommendations of National Law in force.