Problems arose with the Silurian and Sea Devil costumes. The actors within experienced ventilation issues, exacerbated by the heat wave afflicting England at the time. Furthermore, the Silurian outfits had a tendency to buckle around the neck, and this resulted in the mask being improperly fitted to the body on some occasions. The Sea Devil costumes, with their large webbed feet, hindered mobility, and the helmets were too heavy, making the heads tilt to one side and cause the actors' vision to be obscured.
The Myrka was the source of numerous problems. was supposed to be ready for the first studio day, to give actors John Asquith and William Perrie an opportunity to rehearse. The Myrka had been designed in the manner of a pantomime horse, with two operators positioned in either end of the costume. However, Nathan-Turner was informed that not only would the Myrka be unavailable for rehearsals, but the costume would not be physically ready. Nathan-Turner informed Visual Effects that he had no choice but to use the Myrka, regardless of its condition. The paint was still drying, soiling the outfits of those actors with whom the Myrka came in contact. Furthermore, Asquith and Perrie found the costume very difficult to manipulate, and consequently the Myrka was not nearly as mobile as anticipated. This resulted in many of the scenes involving the monster coming across as very awkwardly staged. It is regarded as one of Doctor Who (1963)'s least impressive monsters.